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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 |
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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 |  |  |  |  |  |  |  |  |  |  |  |

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