Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance, Amount at Dec. 31, 2016	$ 118,700	$ 36	$ 365,444	$ (864)	$ (245,916)
Beginning Balance, Shares at Dec. 31, 2016		35,784
Issuance of common stock under stock plans, net of shares withheld for employee taxes	7,921	$ 2	7,919
Issuance of common stock under stock plans, net of shares withheld for employee taxes, Shares		1,719
Stock-based compensation	48,933		48,933
Cumulative adjustment from adoption of stock compensation standard			452		(452)
Unrealized gain (loss) on investments, net of income taxes of 0, 0 and 156	(161)			(161)
Cumulative translation adjustment	968			968
Equity component of 2022 Notes (Note 7)	73,713		73,713
Net loss	(39,714)				(39,714)
Ending Balance, Amount at Dec. 31, 2017	210,360	$ 38	496,461	(57)	(286,082)
Ending Balance, Shares at Dec. 31, 2017		37,503
Issuance of common stock under stock plans, net of shares withheld for employee taxes	14,731	$ 2	14,729
Issuance of common stock under stock plans, net of shares withheld for employee taxes, Shares		1,797
Stock-based compensation	78,518		78,518
Cumulative adjustment from adoption of revenue recognition standard (Note 2)	5,526				5,526
Unrealized gain (loss) on investments, net of income taxes of 0, 0 and 156	110			110
Cumulative translation adjustment	(776)			(776)
Net loss	(63,828)				(63,828)
Ending Balance, Amount at Dec. 31, 2018	244,641	$ 40	589,708	(723)	(344,384)
Ending Balance, Shares at Dec. 31, 2018		39,300
Issuance of common stock under stock plans, net of shares withheld for employee taxes	16,861	$ 2	16,859
Issuance of common stock under stock plans, net of shares withheld for employee taxes, Shares		1,504
Stock-based compensation	99,185		99,185
Issuance of common stock in relation to common stock offering, net of offering costs incurred $365	342,630	$ 2	342,628
Issuance of common stock in relation to common stock offering, net of offering costs incurred $365, Shares		2,151
Unrealized gain (loss) on investments, net of income taxes of 0, 0 and 156	600			600
Cumulative translation adjustment	(213)			(213)
Net loss	(53,746)				(53,746)
Ending Balance, Amount at Dec. 31, 2019	$ 649,958	$ 44	$ 1,048,380	$ (336)	$ (398,130)
Ending Balance, Shares at Dec. 31, 2019		42,955