Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009 Y
Current:
Federal	$ 39,867,000	$ (25,377,000)	$ (134,763,000)
State	6,076,000	(388,000)	(2,712,000)
Subtotal	45,943,000	(25,765,000)	(137,475,000)
Deferred:
Federal	202,392,000	132,161,000	223,177,000
State	31,627,000	21,751,000	11,599,000
Subtotal	234,019,000	153,912,000	234,776,000
Amortization of deferred investment tax credit	(602,000)	(627,000)	(633,000)
Total	279,360,000	127,520,000	96,668,000
Depreciation for qualified investments before increase (as a percent)		50.00%
Depreciation for qualified investments after increase (as a percent)	100.00%
Proceeds from Income Tax Refund		$ 123,400,000	$ 115,200,000
Income Taxes:
Net operating loss carryback provision term after change in law (in years)			5
Deduction of intangible drilling costs (as a percent)	84.00%