Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 22, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data (Unaudited)
Operating revenues		$ 743,591	$ 760,295	$ 744,383	$ 723,694	$ 783,400	$ 805,515	$ 829,275	$ 803,489	$ 2,971,963	$ 3,221,679	$ 3,122,592
Operating income		125,305	146,441	144,093	96,419	88,175	126,678	137,925	121,502	512,258	474,280	388,199
Net income		$ 72,527	$ 91,339	$ 88,052	$ 88,181	$ 54,362	$ 75,859	$ 83,046	$ 67,105	$ 340,099	$ 280,372	$ 428,907
Net income per common share:
Basic (in dollars per share)		$ 1.44	$ 1.80	$ 1.72	$ 1.71	$ 1.05	$ 1.46	$ 1.60	$ 1.30	$ 6.68	$ 5.40	$ 8.28
Diluted (in dollars per share)		$ 1.43	$ 1.79	$ 1.71	$ 1.69	$ 1.03	$ 1.43	$ 1.57	$ 1.28	$ 6.62	$ 5.30	$ 8.08
Weighted average common shares:
Basic (in shares)		50,395,000	50,746,000	51,145,000	51,440,000	51,921,000	52,046,000	52,039,000	51,650,000	50,932,000	51,914,000	51,804,000
Diluted (in shares)		50,796,000	51,129,000	51,477,000	52,098,000	53,033,000	52,913,000	52,981,000	52,556,000	51,375,000	52,871,000	53,100,000
Gain on sale of subsidiary	$ 46,500				$ 46,500					$ 46,525
Special items			$ 21,900							$ 21,869