Income Taxes - Provision for Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Current
Federal									$ 6,088	$ 6,324	$ 5,017
State									493	511	1,089
Total									6,581	6,835	6,106
Deferred
Federal									1,606	1,539	1,444
State									120	(181)	(136)
Deferred income tax expense (benefit), total									1,726	1,358	1,308
Total	$ 2,090	$ 2,032	$ 2,205	$ 1,980	$ 2,121	$ 1,961	$ 1,861	$ 2,250	$ 8,307	$ 8,193	$ 7,414