Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 22, 2019	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Data (Unaudited)
Operating revenues		$ 589,638	$ 457,493	$ 350,039	$ 729,936	$ 743,591	$ 760,295	$ 744,383	$ 723,694	$ 2,127,106	$ 2,971,963	$ 3,221,679
Operating income		(27,669)	74,553	(4,403)	66,321	125,305	146,441	144,093	96,419	108,802	512,258	474,280
Net income		$ (46,450)	$ 33,662	$ (25,715)	$ 29,988	$ 72,527	$ 91,339	$ 88,052	$ 88,181	$ (8,515)	$ 340,099	$ 280,372
Net income per common share:
Basic (in dollars per share)		$ (0.93)	$ 0.67	$ (0.51)	$ 0.60	$ 1.44	$ 1.80	$ 1.72	$ 1.71	$ (0.17)	$ 6.68	$ 5.40
Diluted (in dollars per share)		$ (0.93)	$ 0.66	$ (0.51)	$ 0.59	$ 1.43	$ 1.79	$ 1.71	$ 1.69	$ (0.17)	$ 6.62	$ 5.30
Weighted average common shares:
Basic (in shares)		50,181,000	50,181,000	50,140,000	50,277,000	50,395,000	50,746,000	51,145,000	51,440,000	50,195,000	50,932,000	51,914,000
Diluted (in shares)		50,181,000	50,622,000	50,140,000	50,559,000	50,796,000	51,129,000	51,477,000	52,098,000	50,195,000	51,375,000	52,871,000
Gain on sale of subsidiary	$ 46,500								$ 46,500		$ 46,525
Special items									$ 21,900		$ 21,869