Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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	$ 2,645,821	$ 2,901,312	$ 2,515,134	$ 2,962,219	$ 2,928,628	$ 2,869,807	$ 2,685,096	$ 3,202,604	$ 11,024,486	$ 11,686,135	$ 10,914,922
Operating income	441,010	785,812	422,845	350,845	391,250	665,680	397,208	493,992	2,000,512	1,948,130	1,847,555
Net income	$ 209,025	$ 426,463	$ 196,931	$ 152,066	$ 196,339	$ 368,582	$ 195,164	$ 261,221	$ 984,485	$ 1,021,306	$ 948,234
Basic (in dollars per share)	$ 0.41	$ 0.84	$ 0.39	$ 0.30	$ 0.39	$ 0.73	$ 0.39	$ 0.52	$ 1.94	$ 2.03	$ 1.91
Diluted (in dollars per share)	0.41	0.84	0.39	0.30	0.39	0.73	0.39	0.52	1.94	2.03	1.91
Cash dividends declared per common share (in dollars per share)	$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 1.28	$ 1.20	$ 1.11