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Long-Term Debt (Narrative) (Detail) (USD $) In Millions, unless otherwise specified |  |  | 9 Months Ended | 1 Months Ended | 12 Months Ended |  |  |  | 9 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2012 | Oct. 06, 2011 Line of Credit [Member] | Sep. 30, 2012 Unsecured Senior Notes [Member] | Sep. 30, 2012 Amended Credit Facility [Member] | Jul. 02, 2012 2023 Notes [Member] | Dec. 31, 2011 2019 Notes [Member] | Dec. 31, 2011 2021 Notes [Member] | Dec. 31, 2011 2021 and 2019 Notes [Member] | Apr. 03, 2012 Maximum [Member] Amended Credit Facility [Member] | Sep. 30, 2012 Maximum [Member] Amended Credit Facility [Member] EBITDAX [Member] | Oct. 06, 2011 Minimum [Member] Amended Credit Facility [Member] | Sep. 30, 2012 Minimum [Member] Amended Credit Facility [Member] Current Ratio [Member]
Debt Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Senior unsecured debt principal amount |  |  |  |  | $ 400.0 | $ 400.0 | $ 400.0 |  |  |  |  |
Senior unsecured notes stated interest rate |  |  |  |  | 6.875% | 7.25% | 6.50% |  |  |  |  |
Senior unsecured notes payment frequency |  |  | Semi-annually |  |  |  |  |  |  |  |  |
Net proceeds from issuance of notes |  |  | 1,175.8 |  | 392.4 |  |  | 783.4 |  |  |  |
Debt instrument, maturity date |  |  |  | Oct. 06, 2016 | Jan. 15, 2023 | Feb. 01, 2019 | Nov. 01, 2021 |  |  |  |  |
Redemption percentage, maximum |  |  | 35.00% |  |  |  |  |  |  |  |  |
Percentage of minimum outstanding principal amount |  |  | 65.00% |  |  |  |  |  |  |  |  |
Event of default by payment default |  |  | 10.0 |  |  |  |  |  |  |  |  |
Event of default by final judgments |  |  | 10.0 |  |  |  |  |  |  |  |  |
Letters of credit borrowings |  | 1,000 |  |  |  |  |  |  | 500 |  | 350 |
Percentage of the reserve value of assets as determined by reserve reports |  |  |  | 80.00% |  |  |  |  |  |  |  |
Commitment fee of borrowing base capacity not utilized |  |  |  | 0.375% |  |  |  |  |  |  |  |
Maximum debt to EBITDAX ratio |  |  |  |  |  |  |  |  |  | 4 |  |
Minimum current ratio |  |  |  |  |  |  |  |  |  |  |  | 1.0
Letter of credit unused borrowing base capacity |  |  |  | 500 |  |  |  |  |  |  |  |
Deferred financing costs | $ 24.3 |  |  |  |  |  |  |  |  |  |  |

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