Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 464,749	$ 554,926	$ 440,445	$ 390,839	$ 409,652	$ 530,752	$ 422,985	$ 423,829
Operating income	62,964	122,362	68,386	53,569	54,498	117,076	53,132	49,759	$ 307,281	$ 274,465	$ 266,124
Net income	$ 29,077	$ 68,346	$ 32,211	$ 22,523	$ 22,625	$ 61,815	$ 22,576	$ 20,247	$ 152,157	$ 127,263	$ 129,852