Summarized Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 443,655	$ 552,779	$ 492,536	$ 448,400	$ 389,592	$ 481,407	$ 461,831	$ 374,257
Operating income	45,779	118,769	59,000	42,576	43,836	74,653	58,469	33,059	266,124	210,017	225,755
Net income	$ 16,145	$ 66,937	$ 28,035	$ 18,735	$ 19,350	$ 35,037	$ 28,206	$ 12,584	$ 129,852	$ 95,177	$ 106,369