Selected Consolidated Quarterly Financial Data (Unaudited) - Summary of Basic and Diluted Net Income (Loss) per Common Share Available to Common Shareholders (Detail) - $ / shares	3 Months Ended												12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Net income per common share
Net income per common share available to common shareholders - Basic	$ 0.35	$ 0.09	$ 0.38	$ 0.55	$ 0.66	$ (0.48)	$ 0.10	$ 0.97	$ 0.24	$ 0.67	$ 0.23	$ 0.28	$ 1.37	$ 1.34	$ 1.41
Net income per common share available to common shareholders - Diluted	$ 0.35	$ 0.09	$ 0.38	0.54	0.65	(0.48)	0.10	0.96	0.24	0.66	0.23	0.28	$ 1.36	1.32	1.40
As Previously Reported
Net income per common share
Net income per common share available to common shareholders - Basic				0.80	1.04	(0.89)	0.20	2.00	0.49	1.40	0.48	0.62		2.46	3.01
Net income per common share available to common shareholders - Diluted				$ 0.55	$ 0.65	$ (0.89)	$ 0.10	$ 0.96	$ 0.23	$ 0.66	$ 0.23	$ 0.28		$ 1.32	$ 1.40