Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Income Taxes
Net cash paid (received) for taxes	$ 75,900,000	$ (66,300,000)	$ (59,400,000)
Income tax expense (benefit)
Current income tax expense (benefit)	128,209,000	39,554,000	(96,140,000)
Deferred income tax expense	30,418,000	44,306,000	88,922,000
Total income tax expense (benefit)	158,627,000	83,860,000	(7,218,000)
Effective tax rate
Statutory federal tax rate (as a percent)	35.00%	35.00%	35.00%
State income taxes, net of federal benefit (as a percent)	3.50%	3.20%	1.90%
Other permanent differences (as a percent)	(1.10%)	1.10%	2.70%
Effective tax rate (as a percent)	37.40%	39.30%	39.60%
Deferred tax assets
Capitalized start-up costs	29,182,000	29,728,000
Assets capitalized for tax, expensed for books	32,711,000	28,017,000
Accrued expenses	31,824,000	26,122,000
Total deferred tax assets	93,717,000	83,867,000
Deferred tax liabilities
Property, plant and equipment	(471,847,000)	(448,557,000)
Intangible assets	(79,778,000)	(69,455,000)
Other	(6,666,000)	(2,603,000)
Total deferred tax liabilities	(558,291,000)	(520,615,000)
Net deferred tax liability	$ (464,574,000)	$ (436,748,000)