Summary of Significant Accounting Policies - Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Anti-dilutive common equivalent shares excluded from calculation											900
Net income per share:
Net income available to common stockholders	$ 8,463	$ 8,838	$ 14,669	$ 13,915	$ 91,680	$ 11,841	$ 13,932	$ 9,506	$ 45,885	$ 126,959	$ 11,001
Weighted average common shares outstanding used in computing basic income per share									32,286	30,866	29,195
Weighted average common shares used in computing diluted net income per share									34,002	33,436	30,947
Net income per share
Basic	$ 0.26	$ 0.27	$ 0.46	$ 0.43	$ 2.88	$ 0.38	$ 0.46	$ 0.32	$ 1.42	$ 4.11	$ 0.38
Diluted	$ 0.25	$ 0.26	$ 0.43	$ 0.41	$ 2.68	$ 0.35	$ 0.42	$ 0.29	$ 1.35	$ 3.80	$ 0.36
Restricted Stock
Net income per share:
Incremental options and RSUs									1,716	2,570	1,752