Condensed Statements of Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Loans to Related Parties	Accumulated Deficit	IPO	IPO Common Stock	IPO Additional Paid-in Capital
Beginning balance (in shares) at Dec. 31, 2017	39,361,211
Beginning balance at Dec. 31, 2017	$ 148,194
Preferred Stock
Issuance of Series G preferred stock, net of issuance costs (in shares)	112,945
Issuance of Series G preferred stock, net of issuance costs	$ 1,228
Ending balance (in shares) at Mar. 31, 2018	39,474,156
Ending balance at Mar. 31, 2018	$ 149,422
Beginning balance (in shares) at Dec. 31, 2017		5,724,506
Beginning balance at Dec. 31, 2017	(95,913)	$ 1	$ 4,823	$ (951)	$ (99,786)
Common Stock
Net income (loss)	(5,696)				(5,696)
Issuance of common stock through equity incentive plans (in shares)		92,310
Issuance of common stock through equity incentive plans	88		88
Share-based compensation	260		260
Issuance of Series G preferred stock, net of issuance costs	0
Ending balance (in shares) at Mar. 31, 2018		5,816,816
Ending balance at Mar. 31, 2018	$ (101,261)	$ 1	5,171	(951)	(105,482)
Beginning balance (in shares) at Dec. 31, 2017	39,361,211
Beginning balance at Dec. 31, 2017	$ 148,194
Ending balance (in shares) at Sep. 29, 2018	39,486,895
Ending balance at Sep. 29, 2018	$ 149,541
Beginning balance (in shares) at Dec. 31, 2017		5,724,506
Beginning balance at Dec. 31, 2017	(95,913)	$ 1	4,823	(951)	(99,786)
Common Stock
Net income (loss)	(22,434)
Issuance of common stock upon conversion of convertible preferred stock	0
Reclassification of warrant liability to additional paid-in capital in connection with the initial public offering	0
Ending balance (in shares) at Sep. 29, 2018		6,616,705
Ending balance at Sep. 29, 2018	$ (115,692)	$ 1	6,527	0	(122,220)
Beginning balance (in shares) at Mar. 31, 2018	39,474,156
Beginning balance at Mar. 31, 2018	$ 149,422
Preferred Stock
Issuance of Series G preferred stock, net of issuance costs (in shares)	12,739
Issuance of Series G preferred stock, net of issuance costs	$ 121
Ending balance (in shares) at Jun. 30, 2018	39,486,895
Ending balance at Jun. 30, 2018	$ 149,543
Beginning balance (in shares) at Mar. 31, 2018		5,816,816
Beginning balance at Mar. 31, 2018	(101,261)	$ 1	5,171	(951)	(105,482)
Common Stock
Net income (loss)	(7,396)				(7,396)
Issuance of common stock through equity incentive plans (in shares)		624,411
Issuance of common stock through equity incentive plans	783		783
Share-based compensation	450		450
Issuance of Series G preferred stock, net of issuance costs	0
Ending balance (in shares) at Jun. 30, 2018		6,441,227
Ending balance at Jun. 30, 2018	(107,424)	$ 1	6,404	(951)	(112,878)
Preferred Stock
Issuance of Series G preferred stock, net of issuance costs	$ (2)
Ending balance (in shares) at Sep. 29, 2018	39,486,895
Ending balance at Sep. 29, 2018	$ 149,541
Common Stock
Net income (loss)	(9,342)				(9,342)
Issuance of common stock through equity incentive plans (in shares)		224,387
Issuance of common stock through equity incentive plans	257		257
Re-purchase of common stock (in shares)		(48,909)
Re-purchase of common stock	(514)		(514)
Share-based compensation	380		380
Issuance of Series G preferred stock, net of issuance costs	0
Payoff of promissory notes receivable for restricted stock purchase	951			951
Ending balance (in shares) at Sep. 29, 2018		6,616,705
Ending balance at Sep. 29, 2018	$ (115,692)	$ 1	6,527	$ 0	(122,220)
Beginning balance (in shares) at Dec. 31, 2018	41,562,111
Beginning balance at Dec. 31, 2018	$ 199,540
Ending balance (in shares) at Mar. 30, 2019	41,562,111
Ending balance at Mar. 30, 2019	$ 199,540
Beginning balance (in shares) at Dec. 31, 2018		6,951,350
Beginning balance at Dec. 31, 2018	(121,750)	$ 1	7,921		(129,672)
Common Stock
Net income (loss)	(6,649)				(6,649)
Issuance of common stock through equity incentive plans (in shares)		169,583
Issuance of common stock through equity incentive plans	366		366
Share-based compensation	855		855
Ending balance (in shares) at Mar. 30, 2019		7,120,933
Ending balance at Mar. 30, 2019	$ (127,178)	$ 1	9,142		(136,321)
Beginning balance (in shares) at Dec. 31, 2018	41,562,111
Beginning balance at Dec. 31, 2018	$ 199,540
Ending balance (in shares) at Sep. 28, 2019	0
Ending balance at Sep. 28, 2019	$ 0
Beginning balance (in shares) at Dec. 31, 2018		6,951,350
Beginning balance at Dec. 31, 2018	(121,750)	$ 1	7,921		(129,672)
Common Stock
Net income (loss)	(11,991)
Issuance of common stock upon conversion of convertible preferred stock	199,540
Reclassification of warrant liability to additional paid-in capital in connection with the initial public offering	14,421
Ending balance (in shares) at Sep. 28, 2019		60,565,840
Ending balance at Sep. 28, 2019	$ 376,828	$ 6	518,485		(141,663)
Beginning balance (in shares) at Mar. 30, 2019	41,562,111
Beginning balance at Mar. 30, 2019	$ 199,540
Preferred Stock
Issuance of common stock upon conversion of convertible preferred stock (in shares)	(41,562,111)
Issuance of common stock upon conversion of convertible preferred stock	$ (199,540)
Ending balance (in shares) at Jun. 29, 2019	0
Ending balance at Jun. 29, 2019	$ 0
Beginning balance (in shares) at Mar. 30, 2019		7,120,933
Beginning balance at Mar. 30, 2019	(127,178)	$ 1	9,142		(136,321)
Common Stock
Net income (loss)	(9,441)				(9,441)
Issuance of common stock upon conversion of convertible preferred stock (in shares)		41,562,111					11,068,750
Issuance of common stock upon conversion of convertible preferred stock	199,540	$ 4	199,536			$ 252,453	$ 1	$ 252,452
Issuance of common stock upon exercise of common stock warrants (in shares)		214,875
Issuance of common stock upon exercise of common stock warrants	0
Reclassification of warrant liability to additional paid-in capital in connection with the initial public offering	14,421		14,421
Issuance of common stock through equity incentive plans (in shares)		200,852
Issuance of common stock through equity incentive plans	167		167
Share-based compensation	1,823		1,823
Ending balance (in shares) at Jun. 29, 2019		60,167,521
Ending balance at Jun. 29, 2019	$ 331,785	$ 6	477,541		(145,762)
Ending balance (in shares) at Sep. 28, 2019	0
Ending balance at Sep. 28, 2019	$ 0
Common Stock
Net income (loss)	4,099				4,099
Issuance of common stock upon conversion of convertible preferred stock (in shares)							250,000
Issuance of common stock upon conversion of convertible preferred stock						$ 37,450		37,450
Issuance of common stock through equity incentive plans (in shares)		148,319
Issuance of common stock through equity incentive plans	365		365
Share-based compensation	3,129							$ 3,129
Ending balance (in shares) at Sep. 28, 2019		60,565,840
Ending balance at Sep. 28, 2019	$ 376,828	$ 6	$ 518,485		$ (141,663)