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Long-Term Debt - Additional Information (Detail) (USD $)
 3 Months Ended 9 Months Ended 24 Months Ended 9 Months Ended 9 Months Ended 0 Months Ended 12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Senior Unsecured Debt [Member]
Dec. 31, 2012
Senior Unsecured Debt [Member]
Sep. 30, 2013
Second Amended Credit Facility [Member]
Apr. 05, 2013
Second Amended Credit Facility [Member]
Mar. 31, 2013
Second Amended Credit Facility [Member]
Sep. 30, 2013
Current Ratio [Member]
Second Amended Credit Facility [Member]
Sep. 30, 2013
Minimum [Member]
Second Amended Credit Facility [Member]
Sep. 24, 2013
2022 Notes [Member]
Senior Unsecured Debt [Member]
Dec. 31, 2011
2019 Notes [Member]
Senior Unsecured Debt [Member]
Dec. 31, 2011
2021 Notes [Member]
Senior Unsecured Debt [Member]
Dec. 31, 2012
2023 Notes [Member]
Senior Unsecured Debt [Member]
Oct. 02, 2013
Subsequent Event [Member]
Second Amended Credit Facility [Member]
Debt Disclosure [Line Items]                                
Principal debt amount                       $ 1,000,000,000.0 $ 400,000,000.0 $ 400,000,000.0 $ 400,000,000.0  
Stated interest rate                       6.875% 7.25% 6.50% 6.875%  
Debt instrument, maturity date             Apr. 05, 2018         Mar. 15, 2022 Feb. 01, 2019 Nov. 01, 2021 Jan. 15, 2023  
Net proceeds from issuance of notes           1,175,800,000           983,000,000        
Exchange term                       360 days        
Exchange increase in interest rate                       1.00%        
Senior unsecured notes payment frequency         semi-annually                      
Redemption percentage, maximum         35.00%                      
Redemption term         180 days                      
Percentage of minimum outstanding principal amount         65.00%                      
Event of default by payment default ($10.0 million or more)         10,000,000.0                      
Event of default by final judgments (in excess of $10.0 million)         10,000,000.0                      
Line of credit facility, maximum borrowing capacity               1,250,000,000 750,000,000             1,500,000,000
Line of credit facility, current borrowing capacity               900,000,000               1,500,000,000
Line of credit facility, capacity available for specific purpose other than for trade purchases               2,500,000,000 1,000,000,000              
Percentage of the reserve value of assets as determined by reserve reports (at least 80%)             80.00%                  
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            
Line of credit facility, amount outstanding             160,000,000                 605,200,000
Outstanding letters of credit             5,200,000                  
Line of credit facility, remaining borrowing capacity             1,334,800,000                 894,800,000
Weighted average interest rate             2.80%                  
Deferred financing costs 42,300,000   42,300,000                          
Amortization of deferred financing costs $ 1,000,000 $ 800,000 $ 2,900,000 $ 2,100,000