Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	9 Months Ended	65 Months Ended
	Aug. 23, 2019	Oct. 12, 2018	Nov. 30, 2018	Sep. 30, 2019	Sep. 30, 2019
Class Of Stock [Line Items]
Proceeds from common stock offering				$ 54,120	$ 630,800
2018 Sales Agreement [Member]
Class Of Stock [Line Items]
Proceeds from common stock offering			$ 28,500	$ 54,100
Percentage of gross proceeds from common stock as sales agent cash commission		3.00%
Issuance of common stock through at-the-market offering, net of issuance costs, shares			1,659,300	3,351,808
Common stock price per share			$ 18.00	$ 16.75	$ 16.75
Proceeds from common stock offering				$ 14,000	$ 14,000
2018 Sales Agreement [Member] | General and Administrative Expenses [Member]
Class Of Stock [Line Items]
Legal accounting and other fees			$ 400	$ 200
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]
Percentage of gross proceeds from common stock as sales agent cash commission				3.00%
Issuance of common stock through at-the-market offering, net of issuance costs, shares				0
2019 Sales Agreement [Member] | General and Administrative Expenses [Member]
Class Of Stock [Line Items]
Legal accounting and other fees				$ 100
2019 Sales Agreement [Member] | Maximum [Member]
Class Of Stock [Line Items]
Proceeds from common stock offering	$ 150,000