Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Current:
Federal	$ 100,796	$ 3,771	$ 30,837
State	46,758	229	1,434
Subtotal	147,554	4,000	32,271
Deferred:
Federal	51,767	56,551	177,474
State	(23,940)	11,014	29,003
Subtotal	27,827	67,565	206,477
Amortization of deferred investment tax credit	(195)	(104)	(211)
Income tax expense	$ 175,186	$ 71,461	$ 238,537
Deduction of intangible drilling costs (as a percent)	83.00%