Unaudited Quarterly Financial Data - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	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
Total revenue (1)	$ 260,487	$ 244,108	$ 240,321	$ 245,617	$ 254,747	$ 251,414	$ 245,856	$ 244,793
Total cost of revenue (exclusive of depreciation)	135,165	131,073	138,153	131,206	131,871	132,054	132,234	128,963	$ 535,597	$ 525,122	$ 449,820
Operating income (1)(2)(3)(4)	23,675	28,947	19,775	33,159	30,258	33,420	30,338	32,093	105,556	126,109	104,932
Income before income taxes (1)(2)(3)(4)	19,688	22,742	14,250	28,676	24,214	28,821	26,837	25,851	85,356	105,723	86,987
Income tax provision	(6,423)	(9,176)	(3,884)	(7,162)	(1,633)	(7,262)	(7,458)	(6,600)	$ (26,645)	$ (22,953)	$ (20,857)
Net income (1)(2)(3)(4)(5)	$ 13,265	$ 13,566	$ 10,366	$ 21,514	$ 22,581	$ 21,559	$ 19,379	$ 19,251
Basic earnings per common share (1)(2)(3)(4)(5)	$ 0.42	$ 0.42	$ 0.32	$ 0.67	$ 0.71	$ 0.67	$ 0.60	$ 0.60	$ 1.83	$ 2.58	$ 2.04
Diluted earnings per common share (1)(2)(3)(4)(5)	$ 0.41	$ 0.42	$ 0.32	$ 0.66	$ 0.70	$ 0.66	$ 0.60	$ 0.59	$ 1.82	$ 2.55	$ 2.01