Income Taxes (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009 officer
Income Taxes
Federal statutory rate (as a percent)	35.00%
Tax at statutory rate	$ 265,695,000	$ 124,327,000	$ 88,759,000
State income taxes	25,416,000	14,585,000	8,681,000
Incentive or deferred compensation			8,925,000
Federal tax credits and incentives	(660,000)	(600,000)	1,613,000
Regulatory basis differences	(1,251,000)	(2,713,000)	(9,336,000)
Permanent basis differences	(2,411,000)	(1,258,000)	(3,025,000)
Other	(7,429,000)	(6,821,000)	1,051,000
Income tax expense	279,360,000	127,520,000	96,668,000
Effective tax rate (as a percent)	36.80%	35.90%	38.10%
Income tax deduction limit, annual compensation for covered employees	1,000,000
Reduction in tax expense due to regulatory basis differences, gross			9,800,000
Payments awarded under the 2009 shareholder value plan, tax			$ 8,900,000
Number of highly compensated officers other than the principal executive officer or the principal financial officer			3