Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended			3 Months Ended	6 Months Ended	12 Months Ended	74 Months Ended
	Jun. 05, 2020	Jun. 01, 2020	May 30, 2020	May 31, 2020	Aug. 31, 2019	Oct. 31, 2018	Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2020
Class Of Stock [Line Items]
Common stock, par value							$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Proceeds from common stock offering										$ 889,800
Regeneron Pharmaceuticals Inc. [Member]
Class Of Stock [Line Items]
Proceeds from common stock offering								$ 12,580
2020 Stock Purchase Agreement [Member] | Regeneron Pharmaceuticals Inc. [Member]
Class Of Stock [Line Items]
Common stock, par value			$ 0.0001
Cash consideration received on sale of common stock			$ 30,000
Sale of stock, price per share			$ 32.42
Percentage of premium over volume-weighted average trading price of common stock during 30-day period prior to closing of equity transaction			100.00%
2018 Sales Agreement [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares						5,890,648
Common stock price per share						$ 16.98
Proceeds from common stock offering						$ 96,400
Percentage of gross proceeds from common stock as sales agent cash commission						3.00%
2018 Sales Agreement [Member] | General and Administrative Expenses [Member]
Class Of Stock [Line Items]
Legal accounting and other fees						$ 600
2018 Sales Agreement [Member] | Maximum [Member]
Class Of Stock [Line Items]
Proceeds from common stock offering						$ 100,000
2019 Sales Agreement [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares								1,107,100	287,231
Common stock price per share							$ 13.78	$ 13.78	$ 16.48	$ 13.78
Proceeds from common stock offering								$ 14,700	$ 4,400
Percentage of gross proceeds from common stock as sales agent cash commission								3.00%
Proceeds from common stock offering							$ 130,000	$ 130,000		$ 130,000
2019 Sales Agreement [Member] | General and Administrative Expenses [Member]
Class Of Stock [Line Items]
Legal accounting and other fees								100	$ 200
2019 Sales Agreement [Member] | Maximum [Member]
Class Of Stock [Line Items]
Proceeds from common stock offering					$ 150,000
Follow-on Offering [Member]
Class Of Stock [Line Items]
Proceeds from common stock offering								$ 107,732		$ 249,100
Issuance of common stock							107,732
Follow-on Offering [Member] | Underwriting Agreement [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares		6,301,370
Common stock, par value		$ 0.0001
Shares available for future issuance		821,917
Common stock price per share		$ 18.25
Proceeds from common stock offering	$ 107,700
Common stock offering expenses	$ 400
Private Placement [Member] | Regeneron Pharmaceuticals Inc. [Member]
Class Of Stock [Line Items]
Issuance of common stock							$ 12,580
Private Placement [Member] | 2020 Stock Purchase Agreement [Member] | Regeneron Pharmaceuticals Inc. [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares				925,218
Common stock, par value				$ 0.0001
Cash consideration received on sale of common stock				$ 30,000
Sale of stock, price per share				$ 32.42
Percentage of premium over volume-weighted average trading price of common stock during 30-day period prior to closing of equity transaction				100.00%
Issuance of common stock				$ 12,600