Summary of Significant Accounting Policies - Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue recognition
Maximum deferral period from shipment for recognition of systems revenue and related costs									12 months
Incremental shares excluded from the calculation of net loss per share (in shares)										1,165,855
Net income (loss) per share:
Net income (loss) available to common stockholders	$ 3,965	$ 2,151	$ 2,937	$ 1,948	$ 826	$ 6,101	$ 5,883	$ 1,868	$ 11,001	$ 14,678	$ (11,266)
Weighted average common shares outstanding used in computing basic income (loss) per share									29,195,000	28,595,000	27,862,000
Weighted average common shares outstanding used in computing diluted net income (loss) per share									30,947,000	30,229,000	27,862,000
Net income (loss) per share
Basic	$ 0.13	$ 0.07	$ 0.10	$ 0.07	$ 0.03	$ 0.21	$ 0.21	$ 0.07	$ 0.38	$ 0.51	$ (0.40)
Diluted	$ 0.13	$ 0.07	$ 0.10	$ 0.06	$ 0.03	$ 0.20	$ 0.20	$ 0.06	$ 0.36	$ 0.49	$ (0.40)
Restricted Stock
Net income (loss) per share:
Incremental options and RSUs									1,752,000	1,634,000