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Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands | Dec. 31, 2021 | Sep. 30, 2021
--- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | $ 1,946,139 | $ 2,138,259
FHLB and FRB stock | 102,863 | 102,863
Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 1,946,139 | 2,138,259
Held-to-maturity securities | 326,387 | 366,025
Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 1,946,139 | 2,138,259
Held-to-maturity securities | 335,895 | 379,547
Level 1 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 1,880,647 | 2,090,809
Level 1 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 1,880,647 | 2,090,809
Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
FHLB and FRB stock | 102,863 | 102,863
Time deposits | 3,351,984 | 3,434,087
FHLB advances | 1,720,000 | 1,720,000
Level 2 | Carrying Amount | Client swap program hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 5,944 | 10,983
Other liabilities | 5,944 | 10,983
Level 2 | Carrying Amount | Commercial loan fair value hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other liabilities | 1,951 | 2,177
Level 2 | Carrying Amount | Mortgage loan fair value hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 2,632 | 0
Other liabilities | 0 | 1,641
Level 2 | Carrying Amount | Borrowings cash flow hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 48,888 | 42,442
Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
FHLB and FRB stock | 102,863 | 102,863
Time deposits | 3,307,120 | 3,382,206
FHLB advances | 1,672,238 | 1,692,412
Level 2 | Estimated Fair Value | Client swap program hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 5,944 | 10,983
Other liabilities | 5,944 | 10,983
Level 2 | Estimated Fair Value | Commercial loan fair value hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other liabilities | 1,951 | 2,177
Level 2 | Estimated Fair Value | Mortgage loan fair value hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 2,632 | 0
Other liabilities | 0 | 1,641
Level 2 | Estimated Fair Value | Borrowings cash flow hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 48,888 | 42,442
Level 3 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable | 14,592,202 | 13,833,570
Level 3 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable | 14,991,699 | 14,279,725
U.S. government and agency securities | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 57,567 | 61,779
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, at fair value | 57,567 | 61,779
Asset-backed securities | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 959,894 | 1,078,681
Asset-backed securities | Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 959,894 | 1,078,681
Municipal bonds | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 40,149 | 39,984
Municipal bonds | Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 40,149 | 39,984
Corporate debt securities | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 348,186 | 350,988
Corporate debt securities | Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 348,186 | 350,988
Agency pass-through certificates | Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 540,343 | 606,827
Held-to-maturity securities | 326,387 | 366,025
Agency pass-through certificates | Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities, at fair value | 540,343 | 606,827
Held-to-maturity securities | $ 335,895 | $ 379,547

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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 No definition available. + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 22 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=123594938&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=123599499&loc=d3e63345-112809 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -URI http://asc.fasb.org/subtopic&trid=2196772 + Details
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