Stockholders' Equity - Schedule of Changes in Stockholders Equity (Detail) - USD ($) $ in Thousands	3 Months Ended						6 Months Ended	9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Class Of Stock [Line Items]
Beginning balance	$ 494,813	$ 545,873	$ 527,072	$ 351,587	$ 247,759	$ 269,881	$ 527,072	$ 527,072	$ 269,881
Beginning balance, shares			66,234,056				66,234,056	66,234,056
Exercise of stock options	20,365	6,163	$ 13,340	597	1,035	336
Exercise of stock options, shares								2,636,437
Issuance of shares under employee stock purchase plan		970			685
Equity-based compensation	15,410	10,614	6,424	5,400	4,764	4,157
Other comprehensive income (loss)	(161)	(1)	(13)	(124)	(218)	112
Net loss	(71,644)	(68,806)	(46,205)	(27,840)	(32,393)	(31,806)		$ (186,655)	(92,039)
Ending balance	$ 1,107,098	494,813	545,873	329,620	351,587	247,759	$ 494,813	$ 1,107,098	329,620
Ending balance, shares	74,342,178							74,342,178
Follow-on Offering [Member]
Class Of Stock [Line Items]
Issuance of common stock	$ 648,315				107,732
At The Market Offerings [Member]
Class Of Stock [Line Items]
Issuance of common stock			45,255		9,643	5,079
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	$ 7	$ 7	$ 7	$ 6	$ 5	$ 5	$ 7	$ 7	$ 5
Beginning balance, shares	68,331,780	67,890,334	66,234,056	58,724,238	50,602,875	50,198,044	66,234,056	66,234,056	50,198,044
Exercise of stock options, shares	1,227,067	394,801	1,014,569	67,974	83,631	53,579
Vesting of restricted stock units, shares	24,711	26,235
Issuance of shares under employee stock purchase plan, shares		20,410			55,296
Ending balance	$ 7	$ 7	$ 7	$ 6	$ 6	$ 5	$ 7	$ 7	$ 6
Ending balance, shares	74,342,178	68,331,780	67,890,334	58,792,212	58,724,238	50,602,875	68,331,780	74,342,178	58,792,212
Common Stock [Member] | Follow-on Offering [Member]
Class Of Stock [Line Items]
Issuance of common stock					$ 1
Issuance of common stock, shares	4,758,620				6,301,370
Common Stock [Member] | At The Market Offerings [Member]
Class Of Stock [Line Items]
Issuance of common stock, shares			641,709		755,848	351,252
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	$ 1,044,939	$ 1,027,192	$ 962,173	$ 716,503	$ 580,065	$ 570,493	$ 962,173	$ 962,173	$ 570,493
Exercise of stock options	20,365	6,163	13,340	597	1,035	336
Issuance of shares under employee stock purchase plan		970			685
Equity-based compensation	15,410	10,614	6,424	5,400	4,764	4,157
Ending balance	1,729,029	1,044,939	1,027,192	722,500	716,503	580,065	1,044,939	1,729,029	722,500
Additional Paid-In Capital [Member] | Follow-on Offering [Member]
Class Of Stock [Line Items]
Issuance of common stock	648,315				107,731
Additional Paid-In Capital [Member] | At The Market Offerings [Member]
Class Of Stock [Line Items]
Issuance of common stock			45,255		9,643	5,079
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	(13)	(12)	1	155	373	261	1	1	261
Other comprehensive income (loss)	(161)	(1)	(13)	(124)	(218)	112
Ending balance	(174)	(13)	(12)	31	155	373	(13)	(174)	31
Accumulated Deficit [Member]
Class Of Stock [Line Items]
Beginning balance	(550,120)	(481,314)	(435,109)	(365,077)	(332,684)	(300,878)	(435,109)	(435,109)	(300,878)
Net loss	(71,644)	(68,806)	(46,205)	(27,840)	(32,393)	(31,806)
Ending balance	$ (621,764)	$ (550,120)	$ (481,314)	$ (392,917)	$ (365,077)	$ (332,684)	$ (550,120)	$ (621,764)	$ (392,917)