Net Income (Loss) Per Share - Components and Calculation of Basic and Diluted Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income (loss)		$ (10,612)	$ 98,626	$ 100,864
Dividends on preferred shares	[1]	24,000	18,000	0
Net income (loss) attributable to common shareholders		(34,612)	80,626	100,864
Undistributed income allocated to participating securities			(7,742)
Net income (loss) attributable to common shareholders - basic and diluted		$ (34,612)	$ 72,884	$ 100,864
Weighted-average common shares outstanding - basic		68,424,288	68,012,879	60,315,648
Effect of common share equivalents			1,178,160	1,028,615
Weighted-average common shares outstanding - diluted		68,424,288	69,191,039	61,344,263
Net income (loss) per share - basic	[2]	$ (0.51)	$ 1.07	$ 1.67
Net income (loss) per share - diluted	[2]	$ (0.51)	$ 1.05	$ 1.64

[1] Year ended September 30, 2019 amount is composed of $5.0 million in undeclared cumulative Preferred Stock dividends, as well as an additional $19.0 million of Preferred Stock dividends that had been declared and paid as of period end. Year ended September 30, 2018 amount is composed of $5.0 million in undeclared cumulative Preferred Stock dividends, as well as an additional $13.0 million of Preferred Stock dividends that had been declared and paid as of period end. See Note 3 for further discussion.

[2] See Note 5 for detailed calculations and further discussion.