Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		9 Months Ended
	Apr. 30, 2019	Apr. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	May 31, 2017
ATM Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock issuable and sellable under offering agreement			$ 75.0		$ 75.0
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock under public offering, net of issuance costs (in shares)	12,700,000	14,200,000	12,650	14,157
Public offering price of common stock issued (in USD per share)	$ 11.50	$ 16.25
Net proceeds from issuance of common stock, after deducting offering expenses	$ 136.3	$ 215.8