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 revenue	$ 578,351	$ 621,329	$ 584,133	$ 538,143	$ 519,846	$ 574,841	$ 553,421	$ 493,355	$ 2,321,956	$ 2,141,463	$ 1,931,580
Operating income	85,311	135,216	121,835	101,299	120,337	157,219	122,315	109,251	443,661	509,122	355,263
Net income	$ 48,493	$ 81,382	$ 73,084	$ 61,920	$ 74,400	$ 97,114	$ 76,704	$ 69,002	$ 264,879	$ 317,220	$ 225,464
Basic earnings per share (in dollars per share)	$ 0.70	$ 1.17	$ 1.03	$ 0.87	$ 1.04	$ 1.35	$ 1.06	$ 0.94	$ 3.77	$ 4.39	$ 3.10
Diluted earnings per share (in dollars per share)	$ 0.70	$ 1.17	$ 1.03	$ 0.86	$ 1.04	$ 1.35	$ 1.05	$ 0.94	$ 3.76	$ 4.38	$ 3.08