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Long-Term Debt (Narrative) (Details) (USD $) | 3 Months Ended | 12 Months Ended | 0 Months Ended | 3 Months Ended |  |  | 3 Months Ended | 12 Months Ended |  |  |  | 3 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2012 | Dec. 31, 2011 2019 Notes [Member] | Dec. 31, 2011 2021 Notes [Member] | Oct. 06, 2011 Amended Credit Facility [Member] | Mar. 31, 2012 Amended Credit Facility [Member] | Apr. 03, 2012 Amended Credit Facility [Member] | Feb. 02, 2011 Amended Credit Facility [Member] | Mar. 31, 2012 Unsecured Senior Notes [Member] | Dec. 31, 2011 Unsecured Senior Notes [Member] | Jun. 16, 2011 Line Of Credit [Member] | Oct. 06, 2011 Line Of Credit [Member] Maximum [Member] | Mar. 31, 2012 EBITDAX [Member] Maximum [Member] Amended Credit Facility [Member] | Mar. 31, 2012 Current Ratio [Member] Minimum [Member] Amended Credit Facility [Member]
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  | 4.0 |
Minimum current ratio |  |  |  |  |  |  |  |  |  |  |  |  | 1.0
Letters of credit maximum borrowings |  |  |  | $ 350,000,000 |  | $ 500,000,000 | $ 137,500,000 |  |  | $ 600,000,000 | $ 1,000,000,000 |  |
Letter of credit unused borrowing base capacity |  |  |  |  | 350,000,000 |  |  |  |  |  |  |  |
Senior unsecured debt principal amount |  | 400,000,000 | 400,000,000 |  |  |  |  |  |  |  |  |  |
Senior unsecured notes stated interest rate |  | 7.25% | 6.50% |  |  |  |  |  |  |  |  |  |
Debt instrument, maturity date |  | Feb. 01, 2019 | Nov. 01, 2021 | Feb. 26, 2015 | Oct. 06, 2016 |  |  |  |  |  |  |  |
Senior unsecured notes payment frequency |  |  |  |  |  |  |  | semi-annually |  |  |  |  |
Net proceeds from issuance of notes |  |  |  |  |  |  |  |  | 783,400,000 |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |
Deferred financing costs | $ 17,800,000 |  |  |  |  |  |  |  |  |  |  |  |

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