Quarterly report pursuant to Section 13 or 15(d)

Contingencies - Additional Information (Detail)

v2.4.0.8
Contingencies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 9 Months Ended 12 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2013
Cameron Parish Louisiana Claim
Dec. 31, 2012
Cameron Parish Louisiana Claim
Dec. 31, 2009
Royalties
Sep. 30, 2010
Royalties
Sep. 30, 2013
Contingent Liability Recorded
Sep. 30, 2012
Contingent Liability Recorded
Dec. 31, 2012
Contingent Liability Recorded
Loss Contingencies [Line Items]                
Claims paid   $ 1.3 $ 10.0          
Costs incurred related to an insurance claim in dispute 45.3              
Costs anticipated to be incurred related to an insurance claim in dispute 2.1              
Allowable reductions of cash payments       5.3        
Notified disallowed amount in reductions taken by ONRR         4.7      
Expenses related to accrued and settled claims           0.3 8.8  
Liability loss contingency           $ 0.1   $ 1.3