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Long-Term Debt - Additional Information (Detail) (USD $)
 3 Months Ended 6 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Unsecured Debt [Member]
Jun. 30, 2013
Second Amended Credit Facility [Member]
Apr. 05, 2013
Second Amended Credit Facility [Member]
Mar. 31, 2013
Second Amended Credit Facility [Member]
Jun. 30, 2013
Minimum [Member]
Current Ratio [Member]
Second Amended Credit Facility [Member]
Dec. 31, 2011
2019 Notes [Member]
Unsecured Debt [Member]
Dec. 31, 2011
2021 Notes [Member]
Unsecured Debt [Member]
Dec. 31, 2012
2023 Notes [Member]
Unsecured Debt [Member]
Debt Disclosure [Line Items]                        
Senior unsecured debt principal amount                   $ 400,000,000 $ 400,000,000 $ 400,000,000
Senior unsecured notes stated interest rate                   7.25% 6.50% 6.875%
Debt instrument, maturity date           Oct. 06, 2016       Feb. 01, 2019 Nov. 01, 2021 Jan. 15, 2023
Senior unsecured notes payment frequency         semi-annually              
Net proceeds from issuance of notes         1,175,800,000              
Redemption percentage, maximum         35.00%              
Percentage of minimum outstanding principal amount         65.00%              
Event of default by payment default         10,000,000              
Event of default by final judgments         10,000,000              
Percentage of the reserve value of assets as determined by reserve reports           80.00%            
Letter of credit borrowings             1,250,000,000 750,000,000        
Line of credit facility, current borrowing capacity             900,000,000          
Line of credit facility, capacity available for specific purpose other than for trade purchases           2,500,000,000   1,000,000,000        
Commitment fee of borrowing base capacity not utilized           0.375%            
Ratio of EBITDAX to interest expense necessary to maintain covenants           2.5            
Minimum current ratio                 1.0      
Outstanding letters of credit           2,200,000            
Line of credit facility, remaining borrowing capacity           897,800,000            
Deferred financing costs 24,600,000   24,600,000                  
Amortization of deferred financing costs $ 1,000,000 $ 700,000 $ 1,900,000 $ 1,300,000