Quarterly report pursuant to Section 13 or 15(d)

Share-Based Compensation and Cash-Based Incentive Compensation - Additional Information (Details)

v2.4.1.9
Share-Based Compensation and Cash-Based Incentive Compensation - Additional Information (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 12 Months Ended 1 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Mar. 31, 2015
Mar. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Common stock available for award under plans 4,752,766us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant       4,752,766us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant  
Directors Compensation Plan            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of shares granted 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= wti_DirectorsCompensationPlanMember 		         
Common stock available for award under plans 500,564us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= wti_DirectorsCompensationPlanMember 		      500,564us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= wti_DirectorsCompensationPlanMember 		 
Restricted Stock Units (RSUs)            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of shares granted 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		         
Performance based awards, percentage     100.00%wti_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceBasedUnitsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		100.00%wti_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceBasedUnitsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		   
Number of shares forfeited 49,541us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		         
Number of shares vested 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		       
Expected volatility, minimum       30.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		   
Expected volatility, maximum       63.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		   
Shares granted, grant date fair value   $ 19.4wti_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedInPeriodGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		       
Unrecognized share-based compensation expense 13.7us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		      13.7us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember 		 
Recognition period for unrecognized compensation expense 2016-11          
Restricted Stock Units (RSUs) | London Interbank Offered Rate (LIBOR)            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Risk-free interest rate, minimum       0.27%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember 		   
Risk-free interest rate, maximum       0.91%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember 		   
Restricted Stock Units (RSUs) | Minimum            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Expected dividend yield       0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember 		   
Correlation of movement of total shareholder return       (84.00%)wti_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCorrelationOfReturns
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember 		   
Restricted Stock Units (RSUs) | Maximum            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Expected dividend yield       3.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember 		   
Correlation of movement of total shareholder return       95.00%wti_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCorrelationOfReturns
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember 		   
CEO's 2014 Award | Chief Executive Officer            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Common stock, price per share 14.66us-gaap_SaleOfStockPricePerShare
/ us-gaap_AwardTypeAxis
= wti_TwoThousandFourteenAnnualIncentiveCompensationAwardMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember 		      $ 14.66us-gaap_SaleOfStockPricePerShare
/ us-gaap_AwardTypeAxis
= wti_TwoThousandFourteenAnnualIncentiveCompensationAwardMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember 		 
Number of shares issued         37,316us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardTypeAxis
= wti_TwoThousandFourteenAnnualIncentiveCompensationAwardMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember 		 
Performance based awards, percentage         100.00%wti_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceBasedUnitsPercentage
/ us-gaap_AwardTypeAxis
= wti_TwoThousandFourteenAnnualIncentiveCompensationAwardMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember 		 
CEO's 2013 Award | Chief Executive Officer            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Common stock, price per share   $ 14.84us-gaap_SaleOfStockPricePerShare
/ us-gaap_AwardTypeAxis
= wti_TwoThousandThirteenAnnualIncentiveCompensationAwardMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember 		      $ 14.84us-gaap_SaleOfStockPricePerShare
/ us-gaap_AwardTypeAxis
= wti_TwoThousandThirteenAnnualIncentiveCompensationAwardMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember
Number of shares issued           42,547us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod
/ us-gaap_AwardTypeAxis
= wti_TwoThousandThirteenAnnualIncentiveCompensationAwardMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember
Performance based awards, percentage           100.00%wti_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceBasedUnitsPercentage
/ us-gaap_AwardTypeAxis
= wti_TwoThousandThirteenAnnualIncentiveCompensationAwardMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ChiefExecutiveOfficerMember
Restricted Shares            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of shares granted 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember 		0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember 		       
Number of shares forfeited 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember 		0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember 		       
Number of shares vested 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember 		0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember 		       
Unrecognized share-based compensation expense 0.4us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember 		      $ 0.4us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember 		 
Recognition period for unrecognized compensation expense 2017-04