Quarterly Results of Operations (unaudited) - Results (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 Results of Operations (unaudited)
Revenue	$ 116,396	$ 104,482	$ 102,790	$ 86,893	$ 69,358	$ 65,650	$ 64,451	$ 67,521	$ 410,561	$ 266,980	$ 301,495
Gross profit	36,720	39,751	39,062	34,714	26,947	24,104	25,131	23,415	150,247	99,598	101,706
Net income	$ 91,680	$ 11,841	$ 13,932	$ 9,506	$ 3,965	$ 2,151	$ 2,937	$ 1,948	$ 126,959	$ 11,001	$ 14,678
Net income per basic share (in dollars per share)	$ 2.88	$ 0.38	$ 0.46	$ 0.32	$ 0.13	$ 0.07	$ 0.10	$ 0.07	$ 4.11	$ 0.38	$ 0.51
Net income per diluted share (in dollars per share)	$ 2.68	$ 0.35	$ 0.42	$ 0.29	$ 0.13	$ 0.07	$ 0.10	$ 0.06	$ 3.80	$ 0.36	$ 0.49