Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 496,408	$ 506,871	$ 503,732	$ 501,810	$ 499,410	$ 491,355	$ 470,876	$ 404,490	$ 2,008,821	$ 1,866,131	$ 1,562,662
Operating costs and expenses	457,251	475,916	471,736	503,681	466,285	459,864	439,313	373,780
Income before income taxes	35,007	32,982	33,316	(1,395)	34,678	32,930	33,078	32,279	99,910	132,965	120,387
Net income	$ 31,552	$ 26,086	$ 25,814	$ 72	$ 20,186	$ 23,472	$ 22,551	$ 22,017	$ 83,524	$ 88,226	$ 77,396
Basic earnings per common share (in dollars per share)	$ 0.43	$ 0.35	$ 0.35	$ 0.00	$ 0.27	$ 0.32	$ 0.31	$ 0.30	$ 1.13	$ 1.20	$ 1.06
Diluted earnings per common share (in dollars per share)	0.42	0.35	0.35	0.00	0.27	0.31	0.30	0.30	1.12	1.19	1.05
Cash dividends declared per common share (in shares)	$ 0.19625	$ 0.19500	$ 0.19375	$ 0.19250	$ 0.19125	$ 0.19000	$ 0.18875	$ 0.18750	$ 0.77750	$ 0.75750	$ 0.73750