Fair Value Measurements - Schedule of Fair Value Measurements of Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 1,763,305	$ 1,385,650
Fair value loans	835,822	865,386
U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	93,819	167,172
Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,605,616	1,149,707
States and political subdivision securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	62,870	67,775
Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,763,305	1,385,650
Derivatives-assets	25,843	1,911
Derivatives-liabilities	58	2,188
Fair value loans	835,822	865,386
Loan servicing rights	2,674	3,087
Fair value, measurements, recurring \| U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	93,819	167,172
Fair value, measurements, recurring \| Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,605,616	1,149,707
Fair value, measurements, recurring \| States and political subdivision securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	62,870	67,775
Fair value, measurements, recurring \| Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,000	996
Fair value, measurements, recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	93,819	167,172
Derivatives-assets	0	0
Derivatives-liabilities	0	0
Fair value loans	0	0
Loan servicing rights	0	0
Fair value, measurements, recurring \| Level 1 \| U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	93,819	167,172
Fair value, measurements, recurring \| Level 1 \| Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Fair value, measurements, recurring \| Level 1 \| States and political subdivision securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Fair value, measurements, recurring \| Level 1 \| Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Fair value, measurements, recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,668,175	1,217,508
Derivatives-assets	25,843	1,911
Derivatives-liabilities	58	2,188
Fair value loans	835,822	865,386
Loan servicing rights	0	0
Fair value, measurements, recurring \| Level 2 \| U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Fair value, measurements, recurring \| Level 2 \| Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,605,616	1,149,707
Fair value, measurements, recurring \| Level 2 \| States and political subdivision securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	61,559	66,805
Fair value, measurements, recurring \| Level 2 \| Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,000	996
Fair value, measurements, recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,311	970
Derivatives-assets	0	0
Derivatives-liabilities	0	0
Fair value loans	0	0
Loan servicing rights	2,674	3,087
Fair value, measurements, recurring \| Level 3 \| U.S. Treasury securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Fair value, measurements, recurring \| Level 3 \| Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	0	0
Fair value, measurements, recurring \| Level 3 \| States and political subdivision securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,311	970
Fair value, measurements, recurring \| Level 3 \| Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 0	$ 0

Loans Receivable and Written Loan Commitments, Fair Value Option

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value, after the 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 not subject to a master netting arrangement and not elected to be offset.

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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