Fair Value Measurements - Fair Value Measured on Recurring Basis (Details) - USD ($)	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 2,380,563,000	$ 3,049,442,000
Bank owned life insurance	102,496,000	0
Total Financial Assets	4,017,983,000	4,548,660,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	2,380,563,000	3,049,442,000
Interest rate contracts	11,879,000	2,611,000
Total Financial Assets	2,392,442,000	3,052,053,000
Total Financial Liabilities	27,400,000	2,879,000
Level 1 to level 2 transfers	0	0
Level 2 to level 1 transfers	0	0
Level 1 to level 3 transfers	0	0
Level 3 to level 1 transfers	0	0
Level 2 to level 3 transfers	0	0
Level 3 to level 2 transfers	0	0
Fair Value, Measurements, Recurring \| Interest rate contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities	11,879,000	2,611,000
Fair Value, Measurements, Recurring \| Commercial loan hedge
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities	966,000	0
Fair Value, Measurements, Recurring \| Long term borrowing hedge
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities	14,555,000	268,000
Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	101,952,000	101,387,000
Total Financial Assets	101,952,000	101,387,000
Total Financial Liabilities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Interest rate contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Commercial loan hedge
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities	0	0
Fair Value, Measurements, Recurring \| Level 1 \| Long term borrowing hedge
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities	0	0
Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	2,278,611,000	2,948,055,000
Interest rate contracts	11,879,000	2,611,000
Total Financial Assets	2,290,490,000	2,950,666,000
Total Financial Liabilities	27,400,000	2,879,000
Fair Value, Measurements, Recurring \| Level 2 \| Interest rate contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities	11,879,000	2,611,000
Fair Value, Measurements, Recurring \| Level 2 \| Commercial loan hedge
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities	966,000	0
Fair Value, Measurements, Recurring \| Level 2 \| Long term borrowing hedge
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities	14,555,000	268,000
Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Total Financial Assets	0	0
Total Financial Liabilities	0	0
Fair Value, Measurements, Recurring \| Level 3 \| Interest rate contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial liabilities		0
Equity securities \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	101,952,000	101,387,000
Equity securities \| Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	101,952,000	101,387,000
Equity securities \| Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	101,952,000	101,387,000
Equity securities \| Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Equity securities \| Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Obligations of U.S. government \| Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	482,464,000	731,943,000
Obligations of U.S. government \| Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Obligations of U.S. government \| Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	482,464,000	731,943,000
Obligations of U.S. government \| Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Obligations of states and political subdivisions \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	27,123,000	23,681,000
Obligations of states and political subdivisions \| Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	27,123,000	23,681,000
Obligations of states and political subdivisions \| Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Obligations of states and political subdivisions \| Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	27,123,000	23,681,000
Obligations of states and political subdivisions \| Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Obligations of foreign governments \| Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Obligations of foreign governments \| Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Obligations of foreign governments \| Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Obligations of foreign governments \| Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Corporate debt securities \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	505,800,000	509,007,000
Corporate debt securities \| Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	505,800,000	509,007,000
Corporate debt securities \| Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Corporate debt securities \| Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	505,800,000	509,007,000
Corporate debt securities \| Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Agency pass through mortgage-backed securities \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,160,518,000	1,584,508,000
Agency pass through mortgage-backed securities \| Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,160,518,000	1,584,508,000
Agency pass through mortgage-backed securities \| Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Agency pass through mortgage-backed securities \| Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,160,518,000	1,584,508,000
Agency pass through mortgage-backed securities \| Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Other commercial mortgage-backed securities \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	102,706,000	98,916,000
Other commercial mortgage-backed securities \| Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	102,706,000	98,916,000
Other commercial mortgage-backed securities \| Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Other commercial mortgage-backed securities \| Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	102,706,000	98,916,000
Other commercial mortgage-backed securities \| Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Other debt securites \| Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Other debt securites \| Fair Value, Measurements, Recurring \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Other debt securites \| Fair Value, Measurements, Recurring \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Other debt securites \| Fair Value, Measurements, Recurring \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 0	$ 0

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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The carrying amount of a life insurance policy on an officer, executive or employee for which the reporting entity (a bank) is entitled to proceeds from the policy upon death of the insured or surrender of the insurance policy.

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair Value, Assets, Level 1 to Level 3 Transfers, Amount

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Fair Value, Assets, Level 2 to Level 3 Transfers, Amount

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Fair Value, Assets, Level 3 to Level 1 Transfers, Amount

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Fair Value, Assets, Level 3 to Level 2 Transfers, Amount

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