Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	$ 1,255,401	$ 1,266,209
Held-to-maturity securities	1,610,034	1,635,913
FHLB and FRB stock	128,790	122,990
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,255,401	1,266,209
Held-to-maturity securities	1,670,450	1,646,856
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,255,401	1,266,209
Held-to-maturity securities	1,610,034	1,635,913
Level 1 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	345,919	313,070
Level 1 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	345,919	313,070
Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	128,790	122,990
Customer accounts	11,287,473	10,835,008
FHLB advances	2,370,000	2,225,000
Level 2 \| Carrying Amount \| Interest rate contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	10,141	1,139
Other liabilities	10,141	1,139
Level 2 \| Carrying Amount \| Commercial loan hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	3,005	0
Other liabilities	0	174
Level 2 \| Carrying Amount \| Borrowings hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	19,193	0
Other liabilities	0	1,693
Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	128,790	122,990
Customer accounts	10,924,562	10,411,686
FHLB advances	2,365,478	2,266,791
Level 2 \| Estimated Fair Value \| Interest rate contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	10,141	1,139
Other liabilities	10,141	1,139
Level 2 \| Estimated Fair Value \| Commercial loan hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	3,005	0
Other liabilities	0	174
Level 2 \| Estimated Fair Value \| Borrowings hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	19,193	0
Other liabilities	0	1,693
Level 3 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	11,325,971	10,882,622
FDIC indemnification asset	0	8,968
Level 3 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	11,492,022	11,247,586
FDIC indemnification asset	0	8,564
Equity securities \| Level 1 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	491	522
Equity securities \| Level 1 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	491	522
U.S. government and agency securities \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	216,009	211,077
U.S. government and agency securities \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	216,009	211,077
Municipal bonds \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	22,755	26,624
Municipal bonds \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	22,755	26,624
Corporate debt securities \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	184,232	185,298
Corporate debt securities \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	184,232	185,298
Agency pass-through certificates
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,595,034	1,635,913
Agency pass-through certificates \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	828,449	834,297
Held-to-maturity securities	1,655,450	1,646,856
Agency pass-through certificates \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	828,449	834,297
Held-to-maturity securities	1,595,034	1,635,913
Commercial MBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	15,000
Commercial MBS \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	3,465	8,391
Held-to-maturity securities	15,000	0
Commercial MBS \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	3,465	8,391
Held-to-maturity securities	$ 15,000	$ 0

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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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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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 investment in debt security measured at amortized cost (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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