Quarterly Results of Operations (unaudited) - Results (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	Dec. 31, 2015
Quarterly Results of Operations (unaudited)
Revenue	$ 105,683	$ 95,374	$ 119,333	$ 122,185	$ 116,396	$ 104,482	$ 102,790	$ 86,893	$ 442,575	$ 410,561	$ 266,980	$ 266,980
Gross profit	43,567	39,913	49,000	47,156	36,720	39,751	39,062	34,714	179,636	150,247	99,598
Net income	$ 8,463	$ 8,838	$ 14,669	$ 13,915	$ 91,680	$ 11,841	$ 13,932	$ 9,506	$ 45,885	$ 126,959	$ 11,001
Net income per basic share (in dollars per share)	$ 0.26	$ 0.27	$ 0.46	$ 0.43	$ 2.88	$ 0.38	$ 0.46	$ 0.32	$ 1.42	$ 4.11	$ 0.38
Net income per diluted share (in dollars per share)	$ 0.25	$ 0.26	$ 0.43	$ 0.41	$ 2.68	$ 0.35	$ 0.42	$ 0.29	$ 1.35	$ 3.80	$ 0.36