Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Accumulated Other Comprehensive Income [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2015	$ (21,201)			$ 735	$ (21,936)
Beginning balance, shares at Dec. 31, 2015		2,558,755
Conversion of convertible preferred stock	88,557	$ 3		88,554
Conversion of convertible preferred stock, shares		23,481,956
Issuance of units/stock	167,139	$ 1		167,138
Issuance of units/stock, shares		9,955,554
Exercise of stock options	2			2
Exercise of stock options, shares		257
Issuance of shares under employee stock purchase plan	259			259
Issuance of shares under employee stock purchase plan, shares		23,209
Equity-based compensation	6,715			6,715
Equity-based compensation, shares		(1,191)
Net loss	(31,634)				(31,634)
Ending balance at Dec. 31, 2016	209,837	$ 4		263,403	(53,570)
Ending balance, shares at Dec. 31, 2016		36,018,540
Issuance of units/stock	141,000			141,000
Issuance of units/stock, shares		6,250,000
Exercise of stock options	1,156			1,156
Exercise of stock options, shares		141,759
Issuance of shares under employee stock purchase plan	825			825
Issuance of shares under employee stock purchase plan, shares		64,786
Equity-based compensation	15,322			15,322
Equity-based compensation, shares		(90,462)
Net loss	(67,543)				(67,543)
Ending balance at Dec. 31, 2017	$ 300,597	$ 4		421,706	(121,113)
Ending balance, shares at Dec. 31, 2017	42,384,623	42,384,623
Retroactive adjustment to beginning accumulated deficit for adoption of ASU 2014-09 at Dec. 31, 2018	$ 5,431				5,431
Issuance of units/stock	28,548	$ 1		28,547
Issuance of units/stock, shares		1,659,300
Exercise of stock options	$ 10,651			10,651
Exercise of stock options, shares	1,142,944	1,142,944
Issuance of shares under employee stock purchase plan	$ 1,018			1,018
Issuance of shares under employee stock purchase plan, shares		68,865
Equity-based compensation	17,046			17,046
Equity-based compensation, shares		(31,252)
Other comprehensive loss	(28)		$ (28)
Net loss	(85,343)				(85,343)
Ending balance at Dec. 31, 2018	$ 277,920	$ 5	$ (28)	$ 478,968	$ (201,025)
Ending balance, shares at Dec. 31, 2018	45,224,480	45,224,480