Annual report pursuant to Section 13 and 15(d)

Acquisitions (Narrative) (Details)

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Acquisitions (Narrative) (Details) (USD $)
 12 Months Ended 0 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Aug. 10, 2011
Fairway Properties [Member]
Dec. 31, 2011
Fairway Properties [Member]
Dec. 31, 2011
Yellow Rose Properties [Member]
May 11, 2011
Yellow Rose Properties [Member]
A
Apr. 30, 2010
Total E&P USA's [Member]
Dec. 31, 2011
Shell [Member]
Dec. 31, 2010
Shell [Member]
Nov. 04, 2010
Shell [Member]
Dec. 31, 2011
Matterhorn [Member]
Total E&P USA's [Member]
Dec. 31, 2011
Virgo Field [Member]
Total E&P USA's [Member]
Dec. 31, 2011
Tahoe Field [Member]
Shell [Member]
Dec. 31, 2011
Southeast Tahoe Field [Member]
Shell [Member]
Dec. 31, 2011
Droshky Field [Member]
Shell [Member]
Business Acquisition [Line Items]                                
Number of acres of oil and gas leasehold interests acquired, gross             24,500                  
Number of acres of oil and gas leasehold interests acquired, net             21,900                  
Business acquisition, stated purchase price       $ 55,000,000     $ 366,300,000                  
Business acquisition, purchase price         42,900,000   394,400,000 121,300,000     134,200,000          
The increase primary reflects drilling costs             28,100,000                  
Percentage of working interest include in producing interest       64.30%               100.00% 64.00% 70.00% 100.00%  
Decrease net production cash flow, partially offset by plugging and abandonment costs       12,100,000                        
Asset retirement obligation, estimated property and plant         7,812,000 382,000                    
Business acquisition general administrative expenses 1,600,000                              
Revenue from business acquisition, total 64,000,000                              
Business acquisition direct operating expenses 25,500,000                              
Business acquisition depreciation, depletion, amortization and accretion 20,500,000                              
Business acquisition income taxes 6,300,000                              
Net income from business acquisition 11,700,000                              
Purchase price allocation of unevaluated properties for oil and gas interests 81,200,000                              
Incremental transaction expenses related to acquisitions 1,600,000                              
Business acquisition debt instrument, interest rate, minimum 3.10%                              
Business acquisition debt instrument, interest rate, maximum 3.50%                              
Federal statutory income tax rate 35.00% 35.00% 35.00%                          
Business acquisition, settlement adjustments                 $ 5,700,000 $ 139,900,000            
Percentage of royalty interest include in producing interest                               6.25%