Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 1,270,414	$ 1,922,894
FHLB and FRB stock	124,990	117,205
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,270,414	1,922,894
Held-to-maturity securities	1,651,528	1,417,599
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,270,414	1,922,894
Held-to-maturity securities	1,630,745	1,441,556
Level 1 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	359,252	450,368
Level 1 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	359,252	450,368
Level 1 \| Equity securities \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	522	101,824
Level 1 \| Equity securities \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	522	101,824
Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	124,990	117,205
Customer accounts	10,634,278	10,600,852
FHLB advances	2,275,000	2,080,000
Level 2 \| Carrying Amount \| Interest rate contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets - interest rate contracts	1,342	20,895
Other liabilities	1,342	20,895
Level 2 \| Carrying Amount \| Commercial loan hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	154	3,312
Level 2 \| Carrying Amount \| Borrowings hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	2,536	31,347
Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	124,990	117,205
Customer accounts	10,208,813	10,184,321
FHLB advances	2,319,974	2,184,671
Level 2 \| Estimated Fair Value \| Interest rate contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets - interest rate contracts	1,342	20,895
Other liabilities	1,342	20,895
Level 2 \| Estimated Fair Value \| Commercial loan hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	154	3,312
Level 2 \| Estimated Fair Value \| Borrowings hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other liabilities	2,536	31,347
Level 2 \| Agency pass-through certificates \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,651,528	1,417,599
Level 2 \| Agency pass-through certificates \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,630,745	1,441,556
Level 2 \| U.S. government and agency securities \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	222,245	259,351
Level 2 \| U.S. government and agency securities \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	222,245	259,351
Level 2 \| Municipal bonds \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	26,736	27,670
Level 2 \| Municipal bonds \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	26,736	27,670
Level 2 \| Corporate debt securities \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	211,209	461,138
Level 2 \| Corporate debt securities \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	211,209	461,138
Level 2 \| Agency pass-through certificates \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	801,393	993,041
Level 2 \| Agency pass-through certificates \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	801,393	993,041
Level 2 \| Commercial MBS \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	8,309	79,870
Level 2 \| Commercial MBS \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	8,309	79,870
Level 3 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	10,654,425	9,910,920
FDIC indemnification asset	8,745	12,769
Level 3 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	11,046,152	10,414,794
FDIC indemnification asset	$ 9,160	$ 12,095

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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