Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 409,652	$ 530,752	$ 422,985	$ 423,829	$ 443,655	$ 552,779	$ 492,536	$ 448,400
Operating income	54,498	117,076	53,132	49,759	45,779	118,769	59,000	42,576	$ 274,465	$ 266,124	$ 210,017
Net income	$ 22,625	$ 61,815	$ 22,576	$ 20,247	$ 16,145	$ 66,937	$ 28,035	$ 18,735	$ 127,263	$ 129,852	$ 95,177