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	106,000		106,000
Issuance of common stock upon exercise of stock options, net	4,997,000		5,049,000	(52,000)
Issuance of common stock upon exercise of stock options, Shares		1,670
Issuance of common stock pursuant to exercise of common stock warrants, net, Shares		141
Stock-based compensation expense	4,429,000		4,429,000
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
Unrealized gain on short-term investments, net of tax	0
Net loss	(6,634,000)					(6,634,000)
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
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	17,000		17,000
Share repurchases, Shares		(2)
Issuance of common stock pursuant to equityincentive plans, net of taxes	$ 10,091,000		10,091,000
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares		2,135
Issuance of common stock upon exercise of stock options, Shares	1,974
Issuance of common stock in connection with at-the-market offering, net of issuance costs of $2.0 million	$ 98,025,000		98,025,000
Issuance of common stock in connection with at-the-market offering, net of issuance costs of $2.0 million, Shares		1,389
Stock-based compensation expense	17,864,000		17,864,000
Unrealized gain on short-term investments, net of tax	18,000				18,000
Net loss	(9,732,000)					(9,732,000)
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