Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Oct. 31, 2020	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 Information Disclosure [Abstract]
Operating revenue		$ 467,642	$ 436,727	$ 385,086	$ 352,920	$ 352,841	$ 375,571	$ 374,738	$ 337,723	$ 1,642,375	$ 1,440,873	$ 1,436,370
Operating income		120,395	73,183	82,211	55,193	50,365	61,867	1,801	8,002	330,982	122,035	157,370
Equity in earnings/(loss) of affiliates, net of tax		0	971	376	512	58	607	313	(422)	1,859	556	1,493
Net income from continuing operations		86,430	102,467	51,069	24,284	25,545	31,668	(15,570)	(7,873)	264,250	33,770	74,010
Income from discontinued operations, net of tax		6,214	10,679	7,935	9,535	4,537	(8,485)	9,999	9,559	34,363	15,610	47,854
Gain from sale of discontinued operations, net of tax	$ 2,700	2,742	0	0	0					2,742	0	0
Net income		$ 95,386	$ 113,146	$ 59,004	$ 33,819	$ 30,082	$ 23,183	$ (5,571)	$ 1,686	$ 301,355	$ 49,380	$ 121,864
Earnings Per Share, Basic (usd per share)
Net income from continuing operations (usd per share)		$ 1.13	$ 1.29	$ 0.64	$ 0.31	$ 0.32	$ 0.40	$ (0.19)	$ (0.10)	$ 3.36	$ 0.42	$ 0.92
Income from discontinued operations, net of tax (usd per share)		0.08	0.13	0.10	0.12	0.06	(0.11)	0.12	0.12	0.44	0.20	0.59
Gain from sale of discontinued operations, net of tax (usd per share)		0.04	0	0	0					0.03	0	0
Net income (usd per share)		1.25	1.42	0.74	0.43	0.38	0.29	(0.07)	0.02	3.83	0.62	1.51
Earnings Per Share, Diluted (usd per share)
Net income from continuing operations (usd per share)		1.10	1.26	0.63	0.30	0.32	0.39	(0.19)	(0.10)	3.28	0.42	0.90
Income from discontinued operations, net of tax (usd per share)		0.08	0.13	0.10	0.12	0.06	(0.10)	0.12	0.12	0.43	0.19	0.58
Gain from sale of discontinued operations, net of tax (usd per share)		0	0	0	0.03					0.03	0	0
Net income (usd per share)		$ 1.21	$ 1.39	$ 0.73	$ 0.42	$ 0.38	$ 0.29	$ (0.07)	$ 0.02	$ 3.74	$ 0.61	$ 1.48
Weighted-average common shares outstanding:
Basic (in shares)		76,271	79,467	79,403	79,028	79,125	79,761	80,473	80,179	78,542	79,885	80,854
Diluted (in shares)		78,371	81,402	80,646	80,525	80,356	80,914	80,473	81,277	80,495	81,021	82,275