Income Taxes - Income Tax Expense (Benefit) from Continuing Operations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current:
Federal	$ 164,935	$ 100,796	$ 3,771
State	17,136	46,758	229
Subtotal	182,071	147,554	4,000
Deferred:
Federal	38,357	51,767	56,551
State	(6,336)	(23,940)	11,014
Subtotal	32,021	27,827	67,565
Amortization of deferred investment tax credit	0	(195)	(104)
Total income taxes	$ 214,092	$ 175,186	$ 71,461