Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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	$ 2,794,651	$ 3,040,147	$ 2,499,849	$ 2,772,273	$ 2,645,821	$ 2,901,312	$ 2,515,134	$ 2,962,219	$ 11,106,920	$ 11,024,486	$ 11,686,135
Operating income	465,350	827,054	431,581	489,870	441,010	785,812	422,845	350,845	2,213,855	2,000,512	1,948,130
Net income	$ 227,477	$ 457,795	$ 196,795	$ 241,312	$ 209,025	$ 426,463	$ 196,931	$ 152,066	$ 1,123,379	$ 984,485	$ 1,021,306
Basic (in dollars per share)	$ 0.45	$ 0.90	$ 0.39	$ 0.47	$ 0.41	$ 0.84	$ 0.39	$ 0.30	$ 2.21	$ 1.94	$ 2.03
Diluted (in dollars per share)	0.45	0.90	0.39	0.47	0.41	0.84	0.39	0.30	2.21	1.94	2.03
Cash dividends declared per common share (in dollars per share)	$ 0.34	$ 0.34	$ 0.34	$ 0.34	$ 0.32	$ 0.32	$ 0.32	$ 0.32	$ 1.36	$ 1.28	$ 1.20