Consolidated Statements of Convertible Preferred Stock And Stockholders' Equity (Deficit) - USD ($) shares in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Loss	Accumulated Deficit
Balance at Dec. 26, 2015	$ (159,736,000)			$ 17,335,000			$ (177,071,000)
Balance, Shares at Dec. 26, 2015		80,844
Balance at Dec. 26, 2015		$ 213,180,000
Balance, Shares at Dec. 26, 2015			4,531
Issuance of common stock shares upon exercise of common and preferred warrants, Shares			162
Issuance of common stock upon exercise of stock options	438,000			438,000
Issuance of common stock upon exercise of stock options, Shares			126
Vesting of early exercised stock options	26,000			26,000
Stock-based compensation expense	8,206,000			8,206,000
Unrealized loss on short-term investments	0
Net loss	(42,758,000)						(42,758,000)
Balance at Dec. 31, 2016	(193,824,000)			26,005,000			(219,829,000)
Balance, Shares at Dec. 31, 2016		80,844
Balance at Dec. 31, 2016		$ 213,180,000
Balance, Shares at Dec. 31, 2016			4,819
Issuance of common stock shares upon exercise of common and preferred warrants, Shares			2,165
Issuance of common stock upon exercise of stock options	2,087,000		$ 1,000	2,086,000
Issuance of common stock upon exercise of stock options, Shares			963
Share repurchases	(671,000)				$ (671,000)
Share repurchases, Shares			(92)
Vesting of early exercised stock options	62,000			62,000
Stock-based compensation expense	10,953,000			10,953,000
Issuance of common stock pursuant to an acquisition, Shares			108
Issuance of common stock pursuant to an initial public offering, net of issuance costs of $3.1 million	131,646,000		$ 1,000	131,645,000
Issuance of common stock pursuant to an initial public offering, net of issuance costs of $3.1 million, Shares			10,350
Conversion of preferred stock into common stock pursuant to an initial public offering	213,180,000	$ (213,180,000)	$ 8,000	213,172,000
Conversion of preferred stock into common stock pursuant to an initial public offering, Shares		(80,844)	80,844
Reclassification of preferred stock warrants liability to additional paid-in-capital upon conversion to common stock warrants	52,355,000			52,355,000
Unrealized loss on short-term investments	0
Net loss	(63,509,000)						(63,509,000)
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
Share repurchases, Shares			(9)
Vesting of early exercised stock options	239,000			239,000
Issuance of common stock pursuant to equity incentive plans, net of taxes	25,034,000		$ 1,000	25,033,000
Issuance of common stock pursuant to equity incentive plans, net of taxes, Shares			10,481
Issuance of common stock pursuant to exercise of common stock warrants, net, Shares			141
Stock-based compensation expense	37,674,000			37,674,000
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 loss on short-term investments	(17,000)					$ (17,000)
Net loss	(8,857,000)						(8,857,000)
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