Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 2,380,563	$ 3,049,442
Held-to-maturity securities	1,637,420	1,499,218
Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,380,563	3,049,442
Held-to-maturity securities	1,643,216	1,548,265
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	284,049	781,843
Level 1 \| Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	284,049	781,843
Level 1 \| Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	101,952	101,387
Level 1 \| Equity securities \| Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	101,952	101,387
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	107,198	158,839
Customer accounts	10,004,290	9,946,586
FHLB advances and other borrowings	1,938,384	2,054,437
Level 2 \| Interest rate contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	11,879	2,611
Other liabilities	11,879	2,611
Level 2 \| Commercial loan hedge
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	966	0
Level 2 \| Long term borrowing hedge
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	14,555	268
Level 2 \| Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	107,198	158,839
Customer accounts	10,631,703	10,716,928
FHLB advances and other borrowings	1,830,000	1,930,000
Level 2 \| Carrying amount \| Interest rate contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	11,879	2,611
Other liabilities	11,879	2,611
Level 2 \| Carrying amount \| Commercial loan hedge
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	966	0
Level 2 \| Carrying amount \| Long term borrowing hedge
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	14,555	268
Level 2 \| U.S. agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	482,464	731,943
Level 2 \| U.S. agency securities \| Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	482,464	731,943
Level 2 \| Obligations of states and political subdivisions
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	27,123	23,681
Level 2 \| Obligations of states and political subdivisions \| Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	27,123	23,681
Level 2 \| Corporate debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	505,800	509,007
Level 2 \| Corporate debt securities \| Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	505,800	509,007
Level 2 \| Agency pass through mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,160,518	1,584,508
Held-to-maturity securities	1,637,420	1,499,218
Level 2 \| Agency pass through mortgage-backed securities \| Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,160,518	1,584,508
Held-to-maturity securities	1,643,216	1,548,265
Level 2 \| Other commercial mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	102,706	98,916
Level 2 \| Other commercial mortgage-backed securities \| Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	102,706	98,916
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	9,667,750	8,844,532
FDIC indemnification asset	15,522	35,976
Level 3 \| Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	9,170,634	8,324,798
FDIC indemnification asset	$ 16,275	$ 36,860

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

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Fair value of investments in debt securities classified as held-to-maturity.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Fair value portion of investments in Federal Home Loan Bank (FHLBank) stock.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 325

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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Fair value portion of other assets.

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Fair value portion of other liabilities.

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FDIC Indemnification Asset, Fair Value Disclosure

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Federal Home Loan Bank Advances, Long Term and Other Borrowings, Fair Value Disclosure

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