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	$ 1,020,926	$ 1,059,177	$ 909,852	$ 1,057,841	$ 1,030,838	$ 1,044,704	$ 952,521	$ 1,135,450	$ 4,047,796	$ 4,163,513	$ 4,383,149
Operating income	170,197	315,605	180,629	223,190	173,951	315,174	195,176	215,400	889,621	899,701	802,370
Net income	$ 86,666	$ 173,607	$ 87,344	$ 115,874	$ 84,255	$ 173,081	$ 98,500	$ 110,966	$ 463,491	$ 466,802	$ 455,188