Stockholders' Equity - Common Stock - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended
	Jun. 25, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Mar. 31, 2019	Jul. 26, 2018	Mar. 16, 2018
Class Of Stock [Line Items]
Issuance of common stock		130,497,563		130,497,563			124,015,429
Common stock, shares authorized		200,000,000		200,000,000			200,000,000
Certificate of Amendment [Member]
Class Of Stock [Line Items]
Common stock, shares authorized								200,000,000
Minimum [Member]
Class Of Stock [Line Items]
Minimum closing bid price	$ 1,000,000
2018 Sales Agreement [Member]
Class Of Stock [Line Items]
Equity sales agreement expiration date				Feb. 22, 2021
Value of shares sold under equity distribution agreement				$ 18,700,000
Issuance of common stock		17,719,185		17,719,185
2018 Sales Agreement [Member] | Maximum [Member]
Class Of Stock [Line Items]
Common stock value reserved for future issuance		$ 81,300,000		$ 81,300,000
IPO [Member]
Class Of Stock [Line Items]
Securities authorized for offer and sale, amount						$ 100,000,000
At-The-Market Facility [Member]
Class Of Stock [Line Items]
Issuance of common stock from stock options exercises, net, Shares		0		6,087,382
At-The-Market Facility [Member] | 2018 Sales Agreement [Member]
Class Of Stock [Line Items]
Common stock value reserved for future issuance		$ 31,300,000		$ 31,300,000
Issuance of common stock from stock options exercises, net, Shares		0	1,798,384	6,087,382	5,560,514
Value of shares sold under equity distribution agreement		$ 0	$ 1,900,000	$ 5,000,000	$ 6,900,000
At-The-Market Facility [Member] | 2018 Sales Agreement [Member] | Maximum [Member]
Class Of Stock [Line Items]
Common stock value reserved for future issuance									$ 50,000,000