Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Sep. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 2,756,906us-gaap_AvailableForSaleSecurities	$ 3,049,442us-gaap_AvailableForSaleSecurities
Held-to-maturity securities	1,480,477	1,499,218
FHLB & FRB stock	150,918us-gaap_FederalHomeLoanBankStock	158,839us-gaap_FederalHomeLoanBankStock
Bank owned life insurance	100,961us-gaap_BankOwnedLifeInsurance	0us-gaap_BankOwnedLifeInsurance
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,756,906	3,049,442
Held-to-maturity securities	1,479,781	1,548,265
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	675,064	781,843
Bank owned life insurance	100,961	0
Level 1 [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	675,064	781,843
Bank owned life insurance	100,961	0
Level 1 [Member] \| Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	102,277	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	102,277	101,387
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB & FRB stock	150,918	158,839
Customer accounts	10,057,164	9,946,586
FHLB advances	1,962,356	2,054,437
FDIC liability	29,825	28,823
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability	(9,503)	(268)
Level 2 [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB & FRB stock	150,918	158,839
Customer accounts	10,692,625	10,716,928
FHLB advances	1,830,000	1,930,000
FDIC liability	29,825	28,823
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	1,480,477	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	1,479,781	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	534,445	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	534,445	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	24,013	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	24,013	23,681
Level 2 [Member] \| Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	528,564	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	528,564	509,007
Level 2 [Member] \| Agency Pass-through Certificates [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,456,945	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	1,456,945	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	110,662	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	110,662	98,916
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	9,038,242	8,667,771
Covered loans	144,672	176,761
FDIC indemnification asset	22,543	35,976
Level 3 [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	8,420,988	8,148,322
Covered loans	138,005	176,476
FDIC indemnification asset	$ 23,115	$ 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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

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-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.10)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 10

-Article 9

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- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

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- Definition

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.

+ References

No definition available.

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- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611

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- Definition

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).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 325

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6478792&loc=d3e63345-112809

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- Definition

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

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- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Glossary Loan

-URI http://asc.fasb.org/extlink&oid=6517073

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- Definition

FDIC Indemnification Asset, Fair Value Disclosure

+ References

No definition available.

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- Definition

FDIC Liabilities Fair Value Disclosure

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

Loans and Leases Receivable, Covered Loans, Gross

+ References

No definition available.

+ Details

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