Condensed Consolidated Statement of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Income / (Loss)	Accumulated Deficit
Balance at Dec. 31, 2017	$ 152,279,000	$ 10,000	$ 436,278,000	$ (671,000)		$ (283,338,000)
Balance, Shares at Dec. 31, 2017		99,157
Vesting of early exercised stock options	173,000		173,000
Issuance of common stock pursuant to equity incentive plans, net of taxes	17,384,000	$ 1,000	17,435,000	(52,000)
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares		5,763
Issuance of common stock pursuant to exercise of common stock warrants, net, Shares		141
Stock-based compensation expense	9,738,000		9,738,000
Unrealized gain on short-term investments, net of tax	0
Cumulative effect change in accounting principle | Accounting Standards Update 2016-16	(40,000)					(40,000)
Cumulative effect change in accounting principle | Accounting Standards Update 2014-09	38,339,000					38,339,000
Net loss	(6,108,000)					(6,108,000)
Balance at Jun. 30, 2018	211,765,000	$ 11,000	463,624,000	(723,000)		(251,147,000)
Balance, Shares at Jun. 30, 2018		105,061
Balance at Mar. 31, 2018	193,476,000	$ 10,000	445,862,000	(723,000)		(251,673,000)
Balance, Shares at Mar. 31, 2018		100,968
Vesting of early exercised stock options	67,000		67,000
Issuance of common stock pursuant to equity incentive plans, net of taxes	12,387,000	$ 1,000	12,386,000
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares		4,093
Stock-based compensation expense	5,309,000		5,309,000
Unrealized gain on short-term investments, net of tax	0
Net loss	526,000					526,000
Balance at Jun. 30, 2018	211,765,000	$ 11,000	463,624,000	(723,000)		(251,147,000)
Balance, Shares at Jun. 30, 2018		105,061
Balance at Dec. 31, 2018	244,651,000	$ 11,000	499,224,000	(671,000)	$ (17,000)	(253,896,000)
Balance, Shares at Dec. 31, 2018		109,770
Vesting of early exercised stock options	32,000		32,000
Share repurchases, Shares		(2)
Issuance of common stock pursuant to equity incentive plans, net of taxes	19,148,000	$ 1,000	19,147,000
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares		4,002
Issuance of common stock in connection with at-the-market offering, net of issuance costs	179,360,000		179,360,000
Issuance of common stock in connection with at-the-market offering, net of issuance costs, Shares		2,389
Stock-based compensation expense	36,535,000		36,535,000
Unrealized gain on short-term investments, net of tax	21,000				21,000
Net loss	(19,065,000)					(19,065,000)
Balance at Jun. 30, 2019	460,682,000	$ 12,000	734,298,000	(671,000)	4,000	(272,961,000)
Balance, Shares at Jun. 30, 2019		116,159
Balance at Mar. 31, 2019	360,934,000	$ 11,000	625,221,000	(671,000)	1,000	(263,628,000)
Balance, Shares at Mar. 31, 2019		113,292
Vesting of early exercised stock options	15,000		15,000
Issuance of common stock pursuant to equity incentive plans, net of taxes	9,057,000	$ 1,000	9,056,000
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares		1,867
Issuance of common stock in connection with at-the-market offering, net of issuance costs	81,335,000		81,335,000
Issuance of common stock in connection with at-the-market offering, net of issuance costs, Shares		1,000
Stock-based compensation expense	18,671,000		18,671,000
Unrealized gain on short-term investments, net of tax	3,000				3,000
Net loss	(9,333,000)					(9,333,000)
Balance at Jun. 30, 2019	$ 460,682,000	$ 12,000	$ 734,298,000	$ (671,000)	$ 4,000	$ (272,961,000)
Balance, Shares at Jun. 30, 2019		116,159