Condensed Consolidated Statement of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ 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	$ 10	$ 436,278	$ (671)		$ (283,338)
Balance, Shares at Dec. 31, 2017		99,157
Vesting of early exercised stock options	213		213
Issuance of common stock pursuant to equity incentive plans, net of taxes	25,490	$ 1	25,558	(69)
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares		9,501
Issuance of common stock pursuant to exercise of common stock warrants, net, Shares		141
Stock-based compensation expense	21,237		21,237
Unrealized gain (loss) on short-term investments	(7)				$ (7)
Cumulative effect change in accounting principle | Accounting Standards Update 2016-16	(40)					(40)
Cumulative effect change in accounting principle | Accounting Standards Update 2014-09	38,339					38,339
Net loss	(15,635)					(15,635)
Balance at Sep. 30, 2018	221,876	$ 11	483,286	(740)	(7)	(260,674)
Balance, Shares at Sep. 30, 2018		108,799
Balance at Jun. 30, 2018	211,765	$ 11	463,624	(723)		(251,147)
Balance, Shares at Jun. 30, 2018		105,061
Vesting of early exercised stock options	40		40
Issuance of common stock pursuant to equity incentive plans, net of taxes	8,106		8,123	(17)
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares		3,738
Stock-based compensation expense	11,499		11,499
Unrealized gain (loss) on short-term investments	(7)				(7)
Net loss	(9,527)					(9,527)
Balance at Sep. 30, 2018	221,876	$ 11	483,286	(740)	(7)	(260,674)
Balance, Shares at Sep. 30, 2018		108,799
Balance at Dec. 31, 2018	244,651	$ 11	499,224	(671)	(17)	(253,896)
Balance, Shares at Dec. 31, 2018		109,770
Vesting of early exercised stock options	52		52
Share repurchases, Shares		(2)
Issuance of common stock pursuant to equity incentive plans, net of taxes	24,766	$ 1	24,765
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares		5,281
Issuance of common stock in connection with at-the-market offering, net of issuance costs	179,360		179,360
Issuance of common stock in connection with at-the-market offering, net of issuance costs, Shares		2,389
Stock-based compensation expense	59,153		59,153
Unrealized gain (loss) on short-term investments	15				15
Net loss	(44,220)					(44,220)
Balance at Sep. 30, 2019	463,777	$ 12	762,554	(671)	(2)	(298,116)
Balance, Shares at Sep. 30, 2019		117,438
Balance at Jun. 30, 2019	460,682	$ 12	734,298	(671)	4	(272,961)
Balance, Shares at Jun. 30, 2019		116,159
Vesting of early exercised stock options	20		20
Issuance of common stock pursuant to equity incentive plans, net of taxes	5,618		5,618
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares		1,279
Stock-based compensation expense	22,618		22,618
Unrealized gain (loss) on short-term investments	(6)				(6)
Net loss	(25,155)					(25,155)
Balance at Sep. 30, 2019	$ 463,777	$ 12	$ 762,554	$ (671)	$ (2)	$ (298,116)
Balance, Shares at Sep. 30, 2019		117,438