Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Aug. 31, 2020	Dec. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Class Of Stock [Line Items]
Capital stock authorized for issuance			150,000,000
Common stock, shares authorized			145,000,000	145,000,000
Common stock, par value			$ 0.001	$ 0.001
Preferred stock, shares authorized			5,000,000
Preferred stock, par value			$ 0.001
Common stock, shares outstanding			102,376,303	95,506,271
Net proceeds from common stock after deducting advisory fees and offering expenses		$ 250,500,000	$ 250,478,974	$ 60,521,739	$ 56,585,035
Public Offering
Class Of Stock [Line Items]
Shares issued to institutional investors		4,600,000
Aggregate purchase price paid by investors		$ 266,800,000
Net proceeds from common stock after deducting advisory fees and offering expenses		$ 250,500,000
Stock issued, price per share		$ 58.00
ATM Agreement
Class Of Stock [Line Items]
Shares issued to institutional investors			0
ATM Agreement | Maximum
Class Of Stock [Line Items]
Common stock shares value reserved for future issuance	$ 250,000,000
Percentage of commission to sales agent	3.00%
2004 Equity Incentive Plan, 2013 Equity Incentive Plan, and Inducement Grants
Class Of Stock [Line Items]
Common Stock, Share reserve for issuance			8,462,148