Unaudited Quarterly Financial Data - Quarterly Financial Information (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
Total revenues	$ 254,747	$ 251,414	$ 245,856	$ 244,793	$ 247,267	$ 213,055	$ 213,033	$ 201,704	$ 996,810	$ 875,059	$ 789,582
Cost of revenues (exclusive of depreciation, shown separately below)	131,871	132,054	132,234	128,963	130,180	109,052	109,492	101,096	525,122	449,820	394,811
Operating income	30,258	33,420	30,338	32,093	29,425	25,653	24,087	25,767	126,109	104,932	105,685
Income before income taxes	24,214	28,821	26,837	25,851	25,567	20,492	20,724	20,204	105,723	86,987	87,710
Income tax provision	(1,633)	(7,262)	(7,458)	(6,600)	(4,669)	(4,391)	(5,607)	(6,190)	$ (22,953)	$ (20,857)	$ (26,346)
Net income	$ 22,581	$ 21,559	$ 19,379	$ 19,251	$ 20,898	$ 16,101	$ 15,117	$ 14,014
Basic earnings per common share	$ 0.71	$ 0.67	$ 0.60	$ 0.60	$ 0.65	$ 0.50	$ 0.46	$ 0.43	$ 2.58	$ 2.04	$ 1.89
Diluted earnings per common share	$ 0.70	$ 0.66	$ 0.60	$ 0.59	$ 0.64	$ 0.49	$ 0.46	$ 0.42	$ 2.55	$ 2.01	$ 1.87