Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total operating revenues	$ 969,816	$ 991,968	$ 1,012,956	$ 855,796	$ 862,795	$ 904,330	$ 851,771	$ 704,138		$ 3,830,536	$ 3,323,034	$ 2,643,552
Operating income	124,624	124,681	163,938	87,804	136,065	145,125	108,521	(38,797)		501,047	350,914	384,825
Net income	$ 81,214	$ 83,464	$ 114,501	$ 56,076	$ 91,937	$ 97,480	$ 11,254	$ (44,922)		$ 335,255	$ 155,749	$ 415,522
Basic earnings per share	$ 1.19	$ 1.22	$ 1.67	$ 0.82	$ 1.35	$ 1.43	$ 0.16	$ (0.66)		$ 4.90	$ 2.28	$ 6.00
Diluted earnings per share	$ 1.18	$ 1.22	$ 1.67	$ 0.82	$ 1.34	$ 1.42	$ 0.16	$ (0.66)		$ 4.89	$ 2.28	5.99
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)												2.84
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)												$ 2.84