Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Sep. 30, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 2,304,788	$ 2,380,563
FHLB and FRB stock	111,107	107,198
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,304,788	2,380,563
Held-to-maturity securities	1,598,370	1,643,216
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,304,788	2,380,563
Held-to-maturity securities	1,571,288	1,637,420
Level 1 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	305,959	284,049
Level 1 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	305,959	284,049
Level 1 \| Equity securities \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	101,558	101,952
Level 1 \| Equity securities \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	101,558	101,952
Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	111,107	107,198
Other assets - interest rate contracts	9,656	11,879
Customer accounts	10,651,119	10,631,703
FHLB advances	1,928,000	1,830,000
Level 2 \| Carrying Amount \| Interest rate contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	9,656	11,879
Level 2 \| Carrying Amount \| Commercial loan hedge
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	565	966
Level 2 \| Carrying Amount \| Long term borrowing hedge
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	11,759	14,555
Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	111,107	107,198
Other assets - interest rate contracts	9,656	11,879
Customer accounts	9,920,280	10,004,290
FHLB advances	2,012,603	1,938,384
Level 2 \| Estimated Fair Value \| Interest rate contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	9,656	11,879
Level 2 \| Estimated Fair Value \| Commercial loan hedge
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	565	966
Level 2 \| Estimated Fair Value \| Long term borrowing hedge
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	11,759	14,555
Level 2 \| Agency pass-through certificates \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,598,370	1,643,216
Level 2 \| Agency pass-through certificates \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,571,288	1,637,420
Level 2 \| Obligations of U.S. government \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	425,814	482,464
Level 2 \| Obligations of U.S. government \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	425,814	482,464
Level 2 \| Obligations of states and political subdivisions \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	27,232	27,123
Level 2 \| Obligations of states and political subdivisions \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	27,232	27,123
Level 2 \| Corporate debt securities \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	505,406	505,800
Level 2 \| Corporate debt securities \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	505,406	505,800
Level 2 \| Agency pass-through certificates \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,149,087	1,160,518
Level 2 \| Agency pass-through certificates \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,149,087	1,160,518
Level 2 \| Other Commercial MBS \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	95,691	102,706
Level 2 \| Other Commercial MBS \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	95,691	102,706
Level 3 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	9,402,730	9,170,634
FDIC indemnification asset	14,076	16,275
Level 3 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	9,901,403	9,667,750
FDIC indemnification asset	$ 13,271	$ 15,522

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

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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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, 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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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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

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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 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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