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Long-Term Debt (Narrative) (Details) (USD $) | 12 Months Ended | 0 Months Ended |  | 0 Months Ended | 12 Months Ended | 3 Months Ended | 9 Months Ended |  | 12 Months Ended |  |  | 12 Months Ended
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Dec. 31, 2011 | Feb. 02, 2011 2019 Notes [Member] | Feb. 02, 2011 2019 Notes [Member] | Dec. 31, 2011 2019 Notes [Member] | Nov. 10, 2011 2021 Notes [Member] | Dec. 31, 2011 2021 Notes [Member] | Dec. 31, 2011 Amended Credit Facility [Member] | Oct. 05, 2011 Amended Credit Facility [Member] | Jan. 21, 2011 Amended Credit Facility [Member] | Dec. 31, 2011 Prior To January 13, 2013 [Member] 2021 Notes [Member] | Dec. 31, 2011 Prior To February 1, 2014 [Member] 2019 Notes [Member] | Dec. 31, 2011 Prior To February 1, 2015 [Member] 2019 Notes [Member] | Dec. 31, 2011 Twelve-Month Period Beginning On February 1, 2015 [Member] 2019 Notes [Member] | Dec. 31, 2011 Prior To November 1, 2014 [Member] 2021 Notes [Member] | Dec. 31, 2011 Prior To November 1, 2016 [Member] 2021 Notes [Member] | Dec. 31, 2011 Twelve-Month Period Beginning February 1, 2016 [Member] 2019 Notes [Member] | Dec. 31, 2011 Beginning On February 1, 2017 [Member] 2019 Notes [Member] | Dec. 31, 2011 Twelve-Month Period Beginning On November 1, 2016 [Member] 2021 Notes [Member] | Dec. 31, 2011 Twelve-Month Period Beginning On November 1, 2017 [Member] 2021 Notes [Member] | Dec. 31, 2011 Twelve-Month Period Beginning On November 1, 2018 [Member] 2021 Notes [Member] | Dec. 31, 2011 Beginning On November 1, 2019 [Member] 2021 Notes [Member] | Dec. 31, 2011 Line Of Credit [Member] | Jun. 16, 2011 Line Of Credit [Member] | Dec. 31, 2011 EBITDAX [Member] Maximum [Member] Amended Credit Facility [Member] | Dec. 31, 2011 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 |  | $ 137,500,000 |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000,000,000 | $ 600,000,000 |  |
Letter of credit unused borrowing base capacity |  |  |  |  |  |  | 350,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior unsecured debt principal amount |  | 400,000,000 | 400,000,000 |  | 400,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior unsecured notes stated interest rate |  | 7.25% | 7.25% |  | 6.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, maturity date |  | Feb. 01, 2019 |  |  | Nov. 01, 2021 |  | Oct. 06, 2016 | Feb. 26, 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior unsecured notes payment frequency |  |  | semi-annually |  | semi-annually |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from issuance of notes |  | 390,000,000 |  |  | 393,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Redemption percentage, maximum |  |  |  |  |  |  |  |  |  |  | 35.00% |  |  | 35.00% |  |  |  |  |  |  |  |  |  |  |
Percentage of repurchase price to aggregate principal amount of notes repurchased plus any accrued and unpaid interest |  |  |  |  |  |  |  |  |  | 110.00% | 107.25% | 100.00% | 103.625% | 106.50% | 100.00% | 101.813% | 100.00% | 103.25% | 102.167% | 101.083% | 100.00% |  |  |  |
Percentage of minimum outstanding principal amount |  |  |  |  |  |  |  |  |  |  | 65.00% |  |  | 65.00% |  |  |  |  |  |  |  |  |  |  |
Debt instrument covenant description |  | $10.0 million or more |  |  | $10.0 million or more | 10.0 million |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred financing cost |  |  |  | $ 8,600,000 |  | $ 7,100,000 | $ 2,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Description of minimum financial levels (for example, tangible net worth and working capital) and achievement of certain financial ratios (for example, working capital ratio and debt service coverage ratio), and adherence to certain clauses which generally require or restrict certain actions (for example, entering into a debt arrangement with equal or greater seniority, and selling or discontinuing a certain business segment or material subsidiary) to be in compliance with the covenant clauses of the debt agreement. May also include a discussion of the adverse consequences that would result if the entity violates or fails to satisfy the covenants. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 129 -Paragraph 2, 4 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823 + Details
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