Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 499,410	$ 491,355	$ 470,876	$ 404,490	$ 398,565	$ 392,734	$ 386,556	$ 384,807
Operating costs and expenses	466,285	459,864	439,313	373,780	368,122	363,522	357,875	355,390
Income before income taxes	34,678	32,930	33,078	32,279	30,529	30,571	29,683	29,604	$ 132,965	$ 120,387	$ 89,764
Net income	$ 20,186	$ 23,472	$ 22,551	$ 22,017	$ 20,299	$ 19,711	$ 18,760	$ 18,626	$ 88,226	$ 77,396	$ 58,024
Basic earnings per common share (in dollars per share)	$ 0.27	$ 0.32	$ 0.31	$ 0.30	$ 0.28	$ 0.27	$ 0.26	$ 0.26	$ 1.20	$ 1.06	$ 0.81
Diluted earnings per common share (in dollars per share)	0.27	0.31	0.30	0.30	0.28	0.27	0.26	0.26	1.19	1.05	0.80
Cash dividends declared per common share (in shares)	$ 0.19125	$ 0.19	$ 0.18875	$ 0.1875	$ 0.18625	$ 0.185	$ 0.18375	$ 0.1825	$ 0.7575	$ 0.7375	$ 0.7175