Summary of Significant Accounting Policies - Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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)										4,663,421	3,547,578
Net income (loss) per share
Net income (loss) available to common stockholders	$ 826	$ 6,101	$ 5,883	$ 1,868	$ 164	$ (4,704)	$ (6,900)	$ 174	$ 14,678	$ (11,266)	$ (17,144)
Weighted average common shares outstanding used in computing basic net income (loss) per share									114,378,000	111,450,000	108,869,000
Incremental shares									6,538,000
Weighted average common shares outstanding used in computing diluted net income (loss) per share									120,916,000	111,450,000	108,869,000
Net income (loss) per share
Basic	$ 0.01	$ 0.05	$ 0.05	$ 0.02	$ 0.00	$ (0.04)	$ (0.06)	$ 0.00	$ 0.13	$ (0.10)	$ (0.16)
Diluted	$ 0.01	$ 0.05	$ 0.05	$ 0.02	$ 0.00	$ (0.04)	$ (0.06)	$ 0.00	$ 0.12	$ (0.10)	$ (0.16)