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	$ 1,030,838	$ 1,044,704	$ 952,521	$ 1,135,450	$ 1,136,791	$ 1,049,111	$ 993,704	$ 1,203,543	$ 4,163,513	$ 4,383,149	$ 4,202,628
Operating income	173,951	315,174	195,176	215,400	169,423	261,073	163,437	208,437	899,701	802,370	828,759
Net income	$ 84,255	$ 173,081	$ 98,500	$ 110,966	$ 92,834	$ 154,159	$ 89,792	$ 118,403	$ 466,802	$ 455,188	$ 453,383