Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Sep. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 2,624,374	$ 3,049,442
Held-to-maturity securities	1,553,716	1,499,218
FHLB and FRB stock	103,189	158,839
Bank owned life insurance	101,720	0
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,624,374	3,049,442
Held-to-maturity securities	1,586,514	1,548,265
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	349,550	781,843
Bank owned life insurance	101,720	0
Other assets - interest rate contracts	5,429	2,879
Level 1 [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	349,550	781,843
Bank owned life insurance	101,720	0
Other assets - interest rate contracts	0	0
Level 1 [Member] \| Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	102,447	101,387
Level 1 [Member] \| Equity Securities [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	102,447	101,387
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	103,189	158,839
Customer accounts	9,830,817	9,946,586
FHLB advances	1,833,413	2,054,437
Other liabilities - interest rate contracts	5,429	2,879
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability	3,914	268
Level 2 [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	103,189	158,839
Customer accounts	10,578,385	10,716,928
FHLB advances	1,730,000	1,930,000
Other liabilities - interest rate contracts	0	0
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability	0	0
Level 2 [Member] \| Agency Pass-through Certificates [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,553,716	1,499,218
Level 2 [Member] \| Agency Pass-through Certificates [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,586,514	1,548,265
Level 2 [Member] \| US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	529,472	731,943
Level 2 [Member] \| US Treasury and Government [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	529,472	731,943
Level 2 [Member] \| Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	27,275	23,681
Level 2 [Member] \| Obligations of States and Political Subdivisions [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	27,275	23,681
Level 2 [Member] \| Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	528,716	509,007
Level 2 [Member] \| Corporate Debt Securities [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	528,716	509,007
Level 2 [Member] \| Agency Pass-through Certificates [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,328,852	1,584,508
Level 2 [Member] \| Agency Pass-through Certificates [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,328,852	1,584,508
Level 2 [Member] \| Other Commercial Mortgage-Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	107,612	98,916
Level 2 [Member] \| Other Commercial Mortgage-Backed Securities [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	107,612	98,916
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	9,175,928	8,667,771
Covered loans	81,737	176,761
FDIC indemnification asset	18,263	35,976
Level 3 [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	8,645,609	8,148,322
Covered loans	77,311	176,476
FDIC indemnification asset	$ 18,783	$ 36,860

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 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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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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The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

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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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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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Loans and Leases Receivable, Covered Loans, Gross

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