Stockholders Equity - Schedule of Changes in Stockholders Equity (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Class Of Stock [Line Items]
Beginning balance	$ 264,338	$ 277,920	$ 295,499	$ 300,597	$ 277,920	$ 300,597
Beginning balance, shares		45,224,480			45,224,480
Issuance of common stock through at-the-market offerings, net of issuance costs	31,413	$ 3,639
Exercise of stock options	1,664	360	557	6,720
Exercise of stock options, shares					233,872
Issuance of shares under employee stock purchase plan	534		598
Equity-based compensation	4,404	4,592	4,901	4,107
Other comprehensive gain	196	87
Net loss	(25,683)	(21,940)	(22,219)	(21,356)	$ (47,623)	(43,575)
Ending balance	$ 276,866	264,338	279,336	295,499	$ 276,866	279,336
Ending balance, shares	47,714,575				47,714,575
ASC 842 [Member]
Class Of Stock [Line Items]
Retroactive adjustment to beginning accumulated deficit		(320)
ASU 2014-09 [Member]
Class Of Stock [Line Items]
Retroactive adjustment to beginning accumulated deficit				5,431
Common Stock [Member]
Class Of Stock [Line Items]
Beginning balance	$ 5	$ 5	$ 4	$ 4	$ 5	$ 4
Beginning balance, shares	45,479,098	45,224,480	43,091,922	42,384,623	45,224,480	42,384,623
Issuance of common stock through at-the-market offering, net of issuance costs, shares	1,986,579	223,818
Exercise of stock options, shares	203,072	30,800	68,163	709,321
Issuance of shares under employee stock purchase plan, shares	45,826		32,707
Cancellation of restricted stock, shares			(4,780)	(2,022)
Ending balance	$ 5	$ 5	$ 4	$ 4	$ 5	$ 4
Ending balance, shares	47,714,575	45,479,098	43,188,012	43,091,922	47,714,575	43,188,012
Additional Paid-In Capital [Member]
Class Of Stock [Line Items]
Beginning balance	$ 487,559	$ 478,968	$ 432,533	$ 421,706	$ 478,968	$ 421,706
Issuance of common stock through at-the-market offerings, net of issuance costs	31,413	3,639
Exercise of stock options	1,664	360	557	6,720
Issuance of shares under employee stock purchase plan	534		598
Equity-based compensation	4,404	4,592	4,901	4,107
Ending balance	525,574	487,559	438,589	432,533	525,574	438,589
Accumulated Other Comprehensive Income (Loss) [Member]
Class Of Stock [Line Items]
Beginning balance	59	(28)			(28)
Other comprehensive gain	196	87
Ending balance	255	59			255
Accumulated Deficit [Member]
Class Of Stock [Line Items]
Beginning balance	(223,285)	(201,025)	(137,038)	(121,113)	(201,025)	(121,113)
Net loss	(25,683)	(21,940)	(22,219)	(21,356)
Ending balance	$ (248,968)	(223,285)	$ (159,257)	(137,038)	$ (248,968)	$ (159,257)
Accumulated Deficit [Member] | ASC 842 [Member]
Class Of Stock [Line Items]
Retroactive adjustment to beginning accumulated deficit		$ (320)
Accumulated Deficit [Member] | ASU 2014-09 [Member]
Class Of Stock [Line Items]
Retroactive adjustment to beginning accumulated deficit				$ 5,431