Net Loss Per Share Attributable to Common Stockholders - Schedule of antidilutive securities excluded from computation of earnings per share (Details) - shares	3 Months Ended
	Mar. 30, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)	41,782,880	39,634,923
Convertible preferred stock (as converted)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)	41,562,111	39,474,156
Preferred stock warrants (as converted to common stock warrants and subsequently exercised)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)	160,767	160,767
Common stock warrants (as exercised)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities (in shares)	60,002	0