Summary of Significant Accounting Policies - Loss Per Share of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Basic and diluted net loss per share of common stock:
Net loss	$ (15,409)	$ (13,806)	$ (12,423)	$ (12,483)	$ (10,113)	$ (11,110)	$ (9,504)	$ (5,999)	$ (54,121)	$ (36,726)
Weighted average shares of common stock outstanding (in shares)									54,512,778	40,895,406
Net loss per share of common stock—basic and diluted	$ (0.25)	$ (0.26)	$ (0.24)	$ (0.24)	$ (0.24)	$ (0.27)	$ (0.24)	$ (0.15)	$ (0.99)	$ (0.90)
Antidilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)									32,572,681	5,056,609
Convertible preferred stock
Basic and diluted net loss per share of common stock:
Antidilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)									24,000,000
Restricted stock
Basic and diluted net loss per share of common stock:
Antidilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)									32,400	105,200
Stock options
Basic and diluted net loss per share of common stock:
Antidilutive securities excluded from the computation of diluted weighted average shares outstanding (in shares)									8,540,281	4,951,409