Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities		$ 1,314,957
Held-to-maturity securities	$ 1,511,679	1,548,770
FHLB and FRB stock	134,190	127,190
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,507,937	1,314,957
Held-to-maturity securities	1,508,175	1,625,420
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,507,937	1,314,957
Held-to-maturity securities	1,511,679	1,548,770
Level 1 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	289,828	268,650
Level 1 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	289,828	268,650
Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	134,190	127,190
Time deposit accounts	4,950,320	4,804,803
FHLB advances	2,505,000	2,330,000
Level 2 \| Carrying Amount \| Interest rate contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	13,912	12,731
Other liabilities - interest rate contracts	13,912	12,731
Level 2 \| Carrying Amount \| Commercial loan hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	0	3,857
Other liabilities - interest rate contracts	2,585	0
Level 2 \| Carrying Amount \| Borrowings cash flow hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	0	22,250
Other liabilities - interest rate contracts	4,442	0
Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
FHLB and FRB stock	134,190	127,190
Time deposit accounts	4,959,058	4,779,040
FHLB advances	2,531,425	2,316,964
Level 2 \| Estimated Fair Value \| Interest rate contracts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	13,912	12,731
Other liabilities - interest rate contracts	13,912	12,731
Level 2 \| Estimated Fair Value \| Commercial loan hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	0	3,857
Other liabilities - interest rate contracts	2,585	0
Level 2 \| Estimated Fair Value \| Borrowings cash flow hedges
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	0	22,250
Other liabilities - interest rate contracts	4,442	0
Level 3 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	11,974,533	11,477,081
Level 3 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans receivable	12,485,465	11,556,326
Equity securities \| Level 1 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities		488
Equity securities \| Level 1 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities		488
U.S. government and agency securities \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	248,942	207,293
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	248,942	207,293
Municipal bonds \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	22,885	22,978
Municipal bonds \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	22,885	22,978
Corporate debt securities \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	208,707	184,695
Corporate debt securities \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	208,707	184,695
Agency pass-through certificates
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities		1,533,742
Agency pass-through certificates \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,027,403	896,041
Held-to-maturity securities	1,493,175	1,610,420
Agency pass-through certificates \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	1,027,403	896,041
Held-to-maturity securities	1,496,671	1,533,742
Commercial MBS
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities		15,028
Commercial MBS \| Level 2 \| Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	0	3,462
Held-to-maturity securities	15,000	15,000
Commercial MBS \| Level 2 \| Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	0	3,462
Held-to-maturity securities	$ 15,008	$ 15,028

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

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