Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current:
U.S. federal and state - current									$ 12,363	$ 6,464	$ 4,763
Current tax benefit on equity compensation									(4,427)	6,212	17,522
Total current tax									7,936	12,676	22,285
Deferred:
U.S. federal and state - deferred									(708,149)	244,994	236,330
Total income tax(benefit) provision	$ 8,765	$ 175	$ (754,642)	$ 45,489	$ 74,279	$ 81,713	$ 57,709	$ 43,969	$ (700,213)	$ 257,670	$ 258,615