Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 667,007	$ 687,231	$ 701,682	$ 591,746	$ 578,351	$ 621,329	$ 584,133	$ 538,143	$ 2,647,666	$ 2,321,956	$ 2,141,463
Operating income	92,546	104,104	132,736	59,405	85,311	135,216	121,835	101,299	388,791	443,661	509,122
Net income	$ 250,338	$ 60,190	$ 78,143	$ 31,935	$ 48,493	$ 81,382	$ 73,084	$ 61,920	$ 420,606	$ 264,879	$ 317,220
Basic earnings per share (in dollars per share)	$ 3.64	$ 0.87	$ 1.13	$ 0.46	$ 0.70	$ 1.17	$ 1.03	$ 0.87	$ 6.08	$ 3.77	$ 4.39
Diluted earnings per share (in dollars per share)	$ 3.63	$ 0.87	$ 1.12	$ 0.46	$ 0.70	$ 1.17	$ 1.03	$ 0.86	6.06	$ 3.76	$ 4.38
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017	$ 199,300
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per basic share (in dollars per share)	$ 2.90								2.88
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)	$ 2.89								$ 2.87