Income Taxes (Details) (USD $)	12 Months Ended			3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2011 Tax Year 2006 [Member]	Dec. 31, 2012 Internal Revenue Service (IRS) [Member]	Dec. 31, 2011 Internal Revenue Service (IRS) [Member]	Dec. 31, 2012 State and Local Jurisdiction [Member]	Dec. 31, 2011 State and Local Jurisdiction [Member]
Current portion of income tax expense:
Federal	$ 0	$ 1,757,000	$ (2,903,000)
State	(370,000)	(1,553,000)	(639,000)
Deferred portion of income tax expense	29,638,000	(123,789,000)	(114,517,000)
Income tax benefit (expense)	29,268,000	(123,585,000)	(118,059,000)
Effective tax rate (as a percent)	35.00%	36.50%	37.50%
Excess income tax benefit from the exercise of stock awards	0	0	854,000
Deferred Income Taxes [Abstract]
Deferred Tax Liabilities, Oil and Gas Properties	678,624,000	639,485,000
Deferred Tax Liabilities, Derivatives	15,942,000	13,274,000
Deferred Tax Liabilities, Other	6,443,000	4,129,000
Deferred Tax Liabilities	701,009,000	656,888,000
Deferred Tax Assets, Operating Loss Carryforwards, State and Local	113,522,000	23,651,000
Deferred Tax Assets, Net Profits Plan Liability	29,233,000	40,148,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost	18,026,000	17,728,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits	6,849,000	5,902,000
Tax Credit Carryforward, Amount	5,271,000	4,301,000
Deferred Tax Assets, Other Long-term Liabilities	4,619,000	4,908,000
Deferred Tax Assets, Gross	177,520,000	96,638,000
Deferred Tax Assets, Valuation Allowance	(5,315,000)	(3,791,000)
Deferred Tax Assets, Net	172,205,000	92,847,000
Deferred Tax (Assets) Liabilities, Net	528,804,000	564,041,000
Deferred Tax Liabilities, Gross, Current	(5,442,000)	(3,307,000)
Deferred Tax Assets, Gross, Current	14,021,000	7,529,000
Deferred income taxes	537,383,000	568,263,000
Income Taxes Receivable, Current	3,364,000	5,581,000			2,511,000	5,581,000	853,000	0
Taxes Payable, Current							0	774,000
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	376,600,000
Unrecognized Tax Benefits, Exercise of Stock Awards	93,400,000
Deferred Tax Assets, Operating Loss Carryforwards, State and Local	361,200,000
Deferred Tax Assets, Tax Credit Carryforwards, Research	5,000,000
Deferred Tax Assets, Tax Credit Carryforwards, Other	252,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	29,231,000	(118,652,000)	(110,214,000)
Income Tax Reconciliation, State and Local Income Taxes	992,000	(6,458,000)	(7,750,000)
Income Tax Reconciliation, Tax Credits, Research	970,000	4,516,000	0
Income Tax Reconciliation, Change in Deferred Tax Assets Valuation Allowance	(1,524,000)	(1,627,000)	1,039,000
Income Tax Reconciliation, Nondeductible Expense, Depletion	210,000	341,000	266,000
Income Tax Reconciliation, Other Adjustments	(611,000)	(1,705,000)	(1,400,000)
Amount of refund received from net operating loss carryback claim	22,900,000			5,500,000
Refund Claim		25,000,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	1,961,000	807,000	884,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	0	1,172,000	0
Unrecognized Tax Benefits, Additions For Tax Positions Of Prior Years	317,000	183,000	244,000
Unrecognized Tax Benefits,Reductions For Lapse Of Statute Of Limitations	0	(201,000)	(321,000)
Unrecognized Tax Benefits, Ending Balance	$ 2,278,000	$ 1,961,000	$ 807,000