Accumulated Other Comprehensive Income (Loss) - Summary of Activity in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ 194,153	$ 191,363	$ 191,397	$ 189,508
Ending Balance	198,608	193,666	198,608	193,666
Defined Benefit Pension Plan [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(3,432)	(1,527)	(3,432)	(1,527)
Amounts reclassified from accumulated other comprehensive income (loss)	(87)		(87)
Total other comprehensive income (loss)	(87)		(87)
Ending Balance	(3,519)	(1,527)	(3,519)	(1,527)
Unrealized Gains (Losses) on Securities Available for Sale [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	2,993	616	3,167	807
Other comprehensive income (loss) before reclassifications	(510)	4,497	(684)	4,481
Amounts reclassified from accumulated other comprehensive income (loss)	(329)	(126)	(329)	(301)
Total other comprehensive income (loss)	(839)	4,371	(1,013)	4,180
Ending Balance	2,154	4,987	2,154	4,987
Derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(2,858)	(441)	(3,791)	(560)
Other comprehensive income (loss) before reclassifications	2,936	(2,031)	3,342	(1,858)
Amounts reclassified from accumulated other comprehensive income (loss)	509	8	1,036	(46)
Total other comprehensive income (loss)	3,445	(2,023)	4,378	(1,904)
Ending Balance	587	(2,464)	587	(2,464)
Accumulated Other Comprehensive Income/(Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(3,297)	(1,352)	(4,056)	(1,280)
Other comprehensive income (loss) before reclassifications	2,426	2,466	2,658	2,623
Amounts reclassified from accumulated other comprehensive income (loss)	93	(118)	620	(347)
Total other comprehensive income (loss)	2,519	2,348	3,278	2,276
Ending Balance	$ (778)	$ 996	$ (778)	$ 996