Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders’ (Deficit) Equity - USD ($) $ in Thousands	Total	Common Stock	Treasury Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Series 1 redeemable convertible non-voting preferred stock
Preferred stock, ending balance (in shares) at Dec. 31, 2018							0
Preferred stock, ending balance at Dec. 31, 2018	$ 0						$ 0
Balance, beginning of period (in shares) at Dec. 31, 2017		3,396,264	67,746
Balance, beginning of period at Dec. 31, 2017	84,648	$ 340	$ (5,056)	$ 411,922	$ (322,512)	$ (46)
Increase (Decrease) in Stockholders' Equity
Share-based compensation	$ 13,824			13,824
Exercise of stock options (in shares)	104,445	101,680
Exercise of stock options	$ 3,270	$ 10		3,260
Issuance of common stock - Employee Stock Purchase Plan (in shares)		4,539
Issuance of common stock - Employee Stock Purchase Plan	205			205
Issuance of common stock in a public offering, net (in shares)		920,000
Issuance of common stock in a public offering, net	64,665	$ 92		64,573
Issuance of common stock upon vesting of restricted stock units (in shares)		1,722
Comprehensive loss	(98,134)				(98,036)	(98)
Balance, end of period (in shares) at Dec. 31, 2018		4,424,205	67,746
Balance, end of period at Dec. 31, 2018	68,478	$ 442	$ (5,056)	493,784	(420,548)	(144)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock in public offering, net (in shares)							575,000
Issuance of redeemable convertible preferred stock in public offering, net							$ 22,944
Conversion of redeemable convertible preferred stock into common stock (in shares)		370,000					(37,000)
Conversion of redeemable convertible preferred stock into common stock	1,476	$ 37		1,439			$ (1,476)
Preferred stock, ending balance (in shares) at Dec. 31, 2019							538,000
Preferred stock, ending balance at Dec. 31, 2019	21,468						$ 21,468
Increase (Decrease) in Stockholders' Equity
Share-based compensation	$ 7,338			7,338
Exercise of stock options (in shares)	220	2,985
Exercise of stock options	$ 76	$ 0		76
Issuance of common stock - Employee Stock Purchase Plan (in shares)		8,000
Issuance of common stock - Employee Stock Purchase Plan	98	$ 1		97
Issuance of common stock in a public offering, net (in shares)		259,116
Issuance of common stock in a public offering, net	8,977	$ 26		8,951
Issuance of common stock upon vesting of restricted stock units (in shares)		12,287
Issuance of common stock upon vesting of restricted stock units		$ 1		(1)
Conversion of redeemable convertible preferred stock into common stock (in shares)		370,000					(37,000)
Conversion of redeemable convertible preferred stock into common stock	1,476	$ 37		1,439			$ (1,476)
Comprehensive loss	(112,660)				(112,477)	(183)
Balance, end of period (in shares) at Dec. 31, 2019		5,076,593	67,746
Balance, end of period at Dec. 31, 2019	$ (26,217)	$ 507	$ (5,056)	$ 511,684	$ (533,025)	$ (327)