Stockholders' Equity - Schedule of Changes in Stockholders Equity (Detail) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Class Of Stock [Line Items]
Beginning balance	$ 276,866	$ 264,338	$ 277,920	$ 279,336	$ 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	19,068	31,413	$ 3,639
Exercise of stock options	192	1,664	360	2,180	557	6,720
Exercise of stock options, shares							255,183
Issuance of shares under employee stock purchase plan		534			598
Equity-based compensation	3,079	4,404	4,592	4,456	4,901	4,107
Other comprehensive gain	81	196	87
Net loss	(23,634)	(25,683)	(21,940)	(22,702)	(22,219)	(21,356)	$ (71,257)	(66,277)
Ending balance	$ 275,652	276,866	264,338	263,270	279,336	295,499	$ 275,652	263,270
Ending balance, shares	48,877,297						48,877,297
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	$ 5	$ 4	$ 4	$ 4	$ 5	$ 4
Beginning balance, shares	47,714,575	45,479,098	45,224,480	43,188,012	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	1,141,411	1,986,579	223,818
Exercise of stock options, shares	21,311	203,072	30,800	204,658	68,163	709,321
Issuance of shares under employee stock purchase plan, shares		45,826			32,707
Cancellation of restricted stock, shares				(23,347)	(4,780)	(2,022)
Ending balance	$ 5	$ 5	$ 5	$ 4	$ 4	$ 4	$ 5	$ 4
Ending balance, shares	48,877,297	47,714,575	45,479,098	43,369,323	43,188,012	43,091,922	48,877,297	43,369,323
Additional Paid-In Capital [Member]
Class Of Stock [Line Items]
Beginning balance	$ 525,574	$ 487,559	$ 478,968	$ 438,589	$ 432,533	$ 421,706	$ 478,968	$ 421,706
Issuance of common stock through at-the-market offerings, net of issuance costs	19,068	31,413	3,639
Exercise of stock options	192	1,664	360	2,180	557	6,720
Issuance of shares under employee stock purchase plan		534			598
Equity-based compensation	3,079	4,404	4,592	4,456	4,901	4,107
Ending balance	547,913	525,574	487,559	445,225	438,589	432,533	547,913	445,225
Accumulated Other Comprehensive (Loss) Income [Member]
Class Of Stock [Line Items]
Beginning balance	255	59	(28)				(28)
Other comprehensive gain	81	196	87
Ending balance	336	255	59				336
Accumulated Deficit [Member]
Class Of Stock [Line Items]
Beginning balance	(248,968)	(223,285)	(201,025)	(159,257)	(137,038)	(121,113)	(201,025)	(121,113)
Net loss	(23,634)	(25,683)	(21,940)	(22,702)	(22,219)	(21,356)
Ending balance	$ (272,602)	$ (248,968)	(223,285)	$ (181,959)	$ (159,257)	(137,038)	$ (272,602)	$ (181,959)
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