Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 389,592	$ 481,407	$ 461,831	$ 374,257	$ 347,477	$ 471,889	$ 380,201	$ 340,488
Operating income	43,836	74,653	58,469	33,059	24,510	107,509	61,891	31,845	210,017	225,755	200,597
Net income	$ 19,350	$ 35,037	$ 28,206	$ 12,584	$ 6,468	$ 58,247	$ 30,294	$ 11,360	$ 95,177	$ 106,369	$ 89,901