Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total operating revenues	$ 862,795	$ 904,330	$ 851,771	$ 704,138	$ 666,182	$ 687,227	$ 700,185	$ 589,957	$ 3,323,034	$ 2,643,552	$ 2,320,021
Operating income (loss)	136,065	145,125	108,521	(38,797)	91,296	103,874	131,312	58,342	350,914	384,825	441,458
Net income	$ 91,937	$ 97,480	$ 11,254	$ (44,922)	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 155,749	$ 415,522	$ 263,483
Basic earnings per share (in dollars per share)	$ 1.35	$ 1.43	$ 0.16	$ (0.66)	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 2.28	$ 6.00	$ 3.75
Diluted earnings per share (in dollars per share)	$ 1.34	$ 1.42	$ 0.16	$ (0.66)	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 2.28	5.99	$ 3.74
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017					$ 196,700
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per basic share (in dollars per share)					$ 2.86					2.84
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)					$ 2.85					$ 2.84