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, 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
Revenue recognition
Maximum deferral period from shipment for recognition of systems revenue and related costs									12 months
Anti-dilutive common equivalent shares excluded from calculation										900	900
Net income per share:
Net income available to common stockholders	$ 91,680	$ 11,841	$ 13,932	$ 9,506	$ 3,965	$ 2,151	$ 2,937	$ 1,948	$ 126,959	$ 11,001	$ 14,678
Weighted average common shares outstanding used in computing basic income per share									30,866	29,195	28,595
Weighted average common shares used in computing diluted net income per share									33,436	30,947	30,229
Net income per share
Basic	$ 2.88	$ 0.38	$ 0.46	$ 0.32	$ 0.13	$ 0.07	$ 0.10	$ 0.07	$ 4.11	$ 0.38	$ 0.51
Diluted	$ 2.68	$ 0.35	$ 0.42	$ 0.29	$ 0.13	$ 0.07	$ 0.10	$ 0.06	$ 3.80	$ 0.36	$ 0.49
Restricted Stock
Net income per share:
Incremental options and RSUs									2,570	1,752	1,634