Selected 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 446,960	$ 455,606	$ 462,101	$ 476,111	$ 494,952	$ 505,500	$ 501,587	$ 500,562	$ 1,840,778	$ 2,002,601	$ 1,861,206
Operating costs and expenses	423,556	431,883	439,094	468,366	456,612	474,975	470,405	502,773
(Loss) income before income taxes	25,871	23,716	23,617	11,892	35,007	32,982	33,316	(1,395)	85,096	99,910	132,965
Net income	$ 18,895	$ 18,344	$ 18,186	$ 9,156	$ 31,552	$ 26,086	$ 25,814	$ 72	$ 64,581	$ 83,524	$ 88,226
Basic earnings per common share (in dollars per share)	$ 0.25	$ 0.25	$ 0.24	$ 0.12	$ 0.43	$ 0.35	$ 0.35	$ 0.00	$ 0.87	$ 1.13	$ 1.20
Diluted earnings per common share (in dollars per share)	0.25	0.25	0.24	0.12	0.42	0.35	0.35	0.00	0.87	1.12	1.19
Cash dividends declared per common share (in shares)	$ 0.20125	$ 0.20000	$ 0.19875	$ 0.19750	$ 0.19625	$ 0.19500	$ 0.19375	$ 0.19250	$ 0.79750	$ 0.77750	$ 0.75750