Nature of Operations and Summary of Significant Accounting Policies - Amortization Method and Estimated Lives of Intangible Assets (Details)	12 Months Ended
	Sep. 30, 2020
Core deposit | Minimum
Finite-Lived Intangible Assets [Line Items]
Intangible asset, useful life	5 years
Core deposit | Maximum
Finite-Lived Intangible Assets [Line Items]
Intangible asset, useful life	10 years
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible asset, useful life	13 years
Other intangibles | Minimum
Finite-Lived Intangible Assets [Line Items]
Intangible asset, useful life	1 year 3 months
Other intangibles | Maximum
Finite-Lived Intangible Assets [Line Items]
Intangible asset, useful life	9 years 3 months 29 days