Derivative Financial Instruments - Notional Amounts and Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Positive Fair Value	$ 24,257	$ 1,850
Negative Fair Value	(4,419)	(19,005)
Derivatives Not Designated as Hedging Instruments
Derivative [Line Items]
Notional Amount	1,349,299	1,142,939
Positive Fair Value	24,257	5,630
Negative Fair Value	(4,419)	(22,785)
Derivatives Not Designated as Hedging Instruments \| Mortgage loan commitments
Derivative [Line Items]
Notional Amount	22,195	37,765
Positive Fair Value	0	0
Negative Fair Value	(28)	(48)
Derivatives Not Designated as Hedging Instruments \| Mortgage loan forward sale contracts
Derivative [Line Items]
Notional Amount	27,408	43,628
Positive Fair Value	28	48
Negative Fair Value	0	0
Financial institution counterparties \| Derivatives Not Designated as Hedging Instruments \| Interest rate swaps
Derivative [Line Items]
Notional Amount	1,082,630	1,025,474
Positive Fair Value	22,696	4,967
Negative Fair Value	(2,231)	(22,737)
Customer counterparties \| Derivatives Not Designated as Hedging Instruments \| Interest rate swaps
Derivative [Line Items]
Notional Amount	217,066	36,072
Positive Fair Value	1,533	615
Negative Fair Value	$ (2,160)	$ 0

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section 50
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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