Unaudited Quarterly Financial Data - Quarterly Financial Information (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
Total revenues	$ 247,267	$ 213,055	$ 213,033	$ 201,704	$ 205,204	$ 199,195	$ 192,713	$ 192,470	$ 875,059	$ 789,582	$ 760,958
Total cost of revenues	130,180	109,052	109,492	101,096	99,726	99,717	97,660	97,708	449,820	394,811	375,147
Operating income	29,425	25,653	24,087	25,767	26,048	28,376	24,248	27,013	104,932	105,685	132,629
Income before income taxes	25,567	20,492	20,724	20,204	21,671	23,386	20,303	22,350	86,987	87,710	99,999
Income tax provision	(4,669)	(4,391)	(5,607)	(6,190)	(6,705)	(8,806)	(8,722)	(2,113)	$ (20,857)	$ (26,346)	$ (37,117)
Net income	$ 20,898	$ 16,101	$ 15,117	$ 14,014	$ 14,966	$ 14,580	$ 11,581	$ 20,237
Basic earnings per common share	$ 0.65	$ 0.50	$ 0.46	$ 0.43	$ 0.46	$ 0.45	$ 0.36	$ 0.63	$ 2.04	$ 1.89	$ 2.03
Diluted earnings per common share	$ 0.64	$ 0.49	$ 0.46	$ 0.42	$ 0.45	$ 0.44	$ 0.35	$ 0.62	$ 2.01	$ 1.87	$ 1.90