Consolidated Statements of Redeemable Noncontrolling Interest and Stockholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Redeemable Noncontrolling Interest	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance at Dec. 31, 2017		$ 0
Ending balance at Mar. 31, 2018		0
Beginning balance, shares at Dec. 31, 2017			78,627,369	11,896,882
Beginning balance at Dec. 31, 2017	$ 308,606		$ 499,974	$ 0	$ 4,900	$ (532)	$ (195,736)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options, shares				421,264
Issuance of common stock upon exercise of stock options	1,103				1,103
Stock-based compensation	1,277				1,277
Issuance of common stock upon early exercise of stock options, shares				44,268
Issuance of common stock upon exercise of warrants, shares				31,713
Issuance of common stock upon exercise of warrants	(4)				(4)
Other comprehensive gain (loss), net of tax impact	(298)					(298)
Net loss	(13,844)						(13,844)
Ending balance, shares at Mar. 31, 2018			78,627,369	12,394,127
Ending balance at Mar. 31, 2018	296,848		$ 499,974	$ 0	7,284	(830)	(209,580)
Beginning balance at Dec. 31, 2017		0
Ending balance at Sep. 30, 2018		41,950
Beginning balance, shares at Dec. 31, 2017			78,627,369	11,896,882
Beginning balance at Dec. 31, 2017	308,606		$ 499,974	$ 0	4,900	(532)	(195,736)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive gain (loss), net of tax impact	1
Net loss	(58,991)
Ending balance, shares at Sep. 30, 2018			78,627,369	13,002,822
Ending balance at Sep. 30, 2018	255,187		$ 499,974	$ 0	11,421	(531)	(255,677)
Beginning balance at Mar. 31, 2018		0
Increase (Decrease) in Temporary Equity [Roll Forward]
Fair value adjustment of redeemable noncontrolling interest		41,000
Ending balance at Jun. 30, 2018		41,000
Beginning balance, shares at Mar. 31, 2018			78,627,369	12,394,127
Beginning balance at Mar. 31, 2018	296,848		$ 499,974	$ 0	7,284	(830)	(209,580)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options, shares				148,230
Issuance of common stock upon exercise of stock options	424				424
Stock-based compensation	1,180				1,180
Issuance of common stock upon exercise of warrants, shares				11,922
Issuance of common stock upon exercise of warrants	(2)				(2)
Repurchase of common stock, shares				(31,681)
Repurchase of common stock	(172)				(172)
Other comprehensive gain (loss), net of tax impact	138					138
Net loss	(21,639)						(21,639)
Ending balance, shares at Jun. 30, 2018			78,627,369	12,522,598
Ending balance at Jun. 30, 2018	276,781		$ 499,974	$ 0	8,718	(692)	(231,219)
Increase (Decrease) in Temporary Equity [Roll Forward]
Fair value adjustment of redeemable noncontrolling interest	(950)	950					(950)
Ending balance at Sep. 30, 2018		41,950
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options, shares				250,656
Issuance of common stock upon exercise of stock options	839				839
Stock-based compensation	1,831				1,831
Issuance of common stock upon exercise of warrants, shares				229,568
Issuance of common stock upon exercise of warrants	(33)				(33)
Other comprehensive gain (loss), net of tax impact	161					161
Net loss	(23,508)						(23,508)
Ending balance, shares at Sep. 30, 2018			78,627,369	13,002,822
Ending balance at Sep. 30, 2018	255,187		$ 499,974	$ 0	11,421	(531)	(255,677)
Beginning balance at Dec. 31, 2018		41,800
Increase (Decrease) in Temporary Equity [Roll Forward]
Fair value adjustment of redeemable noncontrolling interest	(4,700)	4,700					(4,700)
Ending balance at Mar. 31, 2019		46,500
Beginning balance, shares at Dec. 31, 2018				85,832,454
Beginning balance at Dec. 31, 2018	483,152			$ 1	764,033	(83)	(280,799)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options, shares				146,318
Issuance of common stock upon exercise of stock options	538				538
Vesting of common stock exercised early	56				56
Common stock issued under employee stock purchase plan (in shares)				119,702
Common stock issued under employee stock purchase plan	1,933				1,933
Stock-based compensation	3,183				3,183
Other comprehensive gain (loss), net of tax impact	416					416
Net loss	(21,351)						(21,351)
Ending balance, shares at Mar. 31, 2019				86,098,474.000
Ending balance at Mar. 31, 2019	468,134			$ 1	771,009	333	(303,209)
Beginning balance at Dec. 31, 2018		41,800
Ending balance at Sep. 30, 2019		46,500
Beginning balance, shares at Dec. 31, 2018				85,832,454
Beginning balance at Dec. 31, 2018	$ 483,152			$ 1	764,033	(83)	(280,799)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options, shares	2,627,486
Other comprehensive gain (loss), net of tax impact	$ 1,192
Net loss	(45,741)
Ending balance, shares at Sep. 30, 2019				93,853,390
Ending balance at Sep. 30, 2019	815,396			$ 1	1,141,885	1,109	(327,599)
Beginning balance at Mar. 31, 2019		46,500
Increase (Decrease) in Temporary Equity [Roll Forward]
Fair value adjustment of redeemable noncontrolling interest	(300)	300					(300)
Ending balance at Jun. 30, 2019		46,800
Beginning balance, shares at Mar. 31, 2019				86,098,474.000
Beginning balance at Mar. 31, 2019	468,134			$ 1	771,009	333	(303,209)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon follow-on offering, net of offering costs of $723	349,709
Issuance of common stock upon exercise of stock options, shares				1,531,672
Issuance of common stock upon exercise of stock options	4,992				4,992
Vesting of restricted stock units (in shares)				1,106
Vesting of common stock exercised early	13				13
Stock-based compensation	3,215				3,215
Other comprehensive gain (loss), net of tax impact	1,011					1,011
Net loss	(11,300)						(11,300)
Ending balance, shares at Jun. 30, 2019				92,806,252
Ending balance at Jun. 30, 2019	815,474			$ 1	1,128,938	1,344	(314,809)
Increase (Decrease) in Temporary Equity [Roll Forward]
Fair value adjustment of redeemable noncontrolling interest	300	(300)					300
Ending balance at Sep. 30, 2019		$ 46,500
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon follow-on offering, (in shares)				949,496
Issuance of common stock upon follow-on offering, net of offering costs of $723					4,265
Issuance of common stock upon exercise of stock options	4,265
Vesting of restricted stock units (in shares)				4,439
Vesting of common stock exercised early	13				13
Common stock issued under employee stock purchase plan (in shares)				93,203
Common stock issued under employee stock purchase plan	3,185				3,185
Stock-based compensation	5,484				5,484
Other comprehensive gain (loss), net of tax impact	(235)					(235)
Net loss	(13,090)						(13,090)
Ending balance, shares at Sep. 30, 2019				93,853,390
Ending balance at Sep. 30, 2019	$ 815,396			$ 1	$ 1,141,885	$ 1,109	$ (327,599)