Stockholders' Equity - Schedule of Changes in Stockholders Equity (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Class Of Stock [Line Items]
Beginning balance	$ 247,759	$ 269,881	$ 264,338	$ 277,920	$ 269,881	$ 277,920	$ 277,920
Beginning balance, shares		50,198,044			50,198,044
Exercise of stock options	1,035	$ 336	1,664	360
Exercise of stock options, shares					137,210
Issuance of shares under employee stock purchase plan	685		534
Equity-based compensation	4,764	4,157	4,404	4,592
Other comprehensive income (loss)	(218)	112	196	87
Net loss	(32,393)	(31,806)	(25,683)	(21,940)	$ (64,199)	(47,623)
Ending balance	$ 351,587	247,759	276,866	264,338	$ 351,587	276,866	$ 269,881
Ending balance, shares	58,724,238				58,724,238		50,198,044
Cumulative Effect, Period of Adoption, Adjustment [Member]
Class Of Stock [Line Items]
Beginning balance				$ (320)		(320)	$ (320)
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201602Member
Follow-on Offering [Member]
Class Of Stock [Line Items]
Issuance of common stock	$ 107,732
At-the-Market Offerings [Member]
Class Of Stock [Line Items]
Issuance of common stock	9,643	5,079	31,413	$ 3,639
Regeneron Pharmaceuticals Inc. [Member] | Private Placement [Member]
Class Of Stock [Line Items]
Issuance of common stock	12,580
Common Stock [Member]
Class Of Stock [Line Items]
Beginning balance	$ 5	$ 5	$ 5	$ 5	$ 5	$ 5	$ 5
Beginning balance, shares	50,602,875	50,198,044	45,479,098	45,224,480	50,198,044	45,224,480	45,224,480
Exercise of stock options, shares	83,631	53,579	203,072	30,800
Issuance of shares under employee stock purchase plan, shares	55,296		45,826
Ending balance	$ 6	$ 5	$ 5	$ 5	$ 6	$ 5	$ 5
Ending balance, shares	58,724,238	50,602,875	47,714,575	45,479,098	58,724,238	47,714,575	50,198,044
Common Stock [Member] | Follow-on Offering [Member]
Class Of Stock [Line Items]
Issuance of common stock	$ 1
Issuance of common stock, shares	6,301,370
Common Stock [Member] | At-the-Market Offerings [Member]
Class Of Stock [Line Items]
Issuance of common stock, shares	755,848	351,252	1,986,579	223,818
Common Stock [Member] | Regeneron Pharmaceuticals Inc. [Member] | Private Placement [Member]
Class Of Stock [Line Items]
Issuance of common stock, shares	925,218
Additional Paid-In Capital [Member]
Class Of Stock [Line Items]
Beginning balance	$ 580,065	$ 570,493	$ 487,559	$ 478,968	$ 570,493	$ 478,968	$ 478,968
Exercise of stock options	1,035	336	1,664	360
Issuance of shares under employee stock purchase plan	685		534
Equity-based compensation	4,764	4,157	4,404	4,592
Ending balance	716,503	580,065	525,574	487,559	716,503	525,574	570,493
Additional Paid-In Capital [Member] | Follow-on Offering [Member]
Class Of Stock [Line Items]
Issuance of common stock	107,731
Additional Paid-In Capital [Member] | At-the-Market Offerings [Member]
Class Of Stock [Line Items]
Issuance of common stock	9,643	5,079	31,413	3,639
Additional Paid-In Capital [Member] | Regeneron Pharmaceuticals Inc. [Member] | Private Placement [Member]
Class Of Stock [Line Items]
Issuance of common stock	12,580
Accumulated Other Comprehensive Income (Loss) [Member]
Class Of Stock [Line Items]
Beginning balance	373	261	59	(28)	261	(28)	(28)
Other comprehensive income (loss)	(218)	112	196	87
Ending balance	155	373	255	59	155	255	261
Accumulated Deficit [Member]
Class Of Stock [Line Items]
Beginning balance	(332,684)	(300,878)	(223,285)	(201,025)	(300,878)	(201,025)	(201,025)
Net loss	(32,393)	(31,806)	(25,683)	(21,940)
Ending balance	$ (365,077)	$ (332,684)	$ (248,968)	(223,285)	$ (365,077)	(248,968)	(300,878)
Accumulated Deficit [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Class Of Stock [Line Items]
Beginning balance				$ (320)		$ (320)	$ (320)
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201602Member