Derivative Financial Instruments - Schedule of Derivative Positions, Notional Amounts and Estimated Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Derivative [Line Items]
Gross Asset Fair Value	$ 84,442	$ 48,700
Gross Liability Fair Value	(69,645)	(38,826)
Not Designated as Hedging Instruments
Derivative [Line Items]
Notional Amount	2,276,136	1,934,978
Gross Asset Fair Value	84,442	48,700
Gross Liability Fair Value	(69,645)	(38,826)
Not Designated as Hedging Instruments \| Interest rate swaps \| Financial institution counterparties
Derivative [Line Items]
Notional Amount	1,279,918	1,259,765
Gross Asset Fair Value	0	35
Gross Liability Fair Value	(68,412)	(38,755)
Not Designated as Hedging Instruments \| Interest rate swaps \| Customer counterparties
Derivative [Line Items]
Notional Amount	574,453	499,643
Gross Asset Fair Value	83,802	48,652
Gross Liability Fair Value	0	0
Not Designated as Hedging Instruments \| Interest rate caps \| Financial institution counterparties
Derivative [Line Items]
Notional Amount	3,438	100
Gross Asset Fair Value	4	2
Gross Liability Fair Value	0	0
Not Designated as Hedging Instruments \| Interest rate caps \| Customer counterparties
Derivative [Line Items]
Notional Amount	3,438	100
Gross Asset Fair Value	0	0
Gross Liability Fair Value	(4)	(2)
Not Designated as Hedging Instruments \| Risk participation agreements
Derivative [Line Items]
Notional Amount	78,194	56,833
Gross Asset Fair Value	0	0
Gross Liability Fair Value	(593)	(58)
Not Designated as Hedging Instruments \| Mortgage loan commitments
Derivative [Line Items]
Notional Amount	170,012	56,665
Gross Asset Fair Value	636	0
Gross Liability Fair Value	0	(11)
Not Designated as Hedging Instruments \| Mortgage loan forward sale contracts
Derivative [Line Items]
Notional Amount	166,683	61,872
Gross Asset Fair Value	0	11
Gross Liability Fair Value	$ (636)	$ 0

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Nominal or face amount used to calculate payment on derivative.

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