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Fair Value Measurements - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Sep. 30, 2017
--- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | $ 1,245,855 | $ 1,266,209
Held-to-maturity securities | 1,753,342 | 1,635,913
FHLB and FRB stock | 130,590 | 122,990
Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 1,245,855 | 1,266,209
Held-to-maturity securities | 1,765,886 | 1,646,856
Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 1,245,855 | 1,266,209
Held-to-maturity securities | 1,753,342 | 1,635,913
Agency pass-through certificates |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Held-to-maturity securities | 1,738,337 | 1,635,913
Commercial MBS |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Held-to-maturity securities | 15,005 |
Level 1 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 309,713 | 313,070
Level 1 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 309,713 | 313,070
Level 1 | Equity securities | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 503 | 522
Level 1 | Equity securities | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 503 | 522
Level 2 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
FHLB and FRB stock | 130,590 | 122,990
Customer accounts | 11,001,579 | 10,835,008
FHLB advances | 2,415,000 | 2,225,000
Level 2 | Carrying Amount | Interest rate contracts |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 7,587 | 1,139
Other liabilities | 7,587 | 1,139
Level 2 | Carrying Amount | Borrowings hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 4,997 | 0
Other liabilities | 0 | 1,693
Level 2 | Carrying Amount | Commercial loan hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other liabilities | 36 | 174
Level 2 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
FHLB and FRB stock | 130,590 | 122,990
Customer accounts | 10,483,114 | 10,411,686
FHLB advances | 2,431,792 | 2,266,791
Level 2 | Estimated Fair Value | Interest rate contracts |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 7,587 | 1,139
Other liabilities | 7,587 | 1,139
Level 2 | Estimated Fair Value | Borrowings hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other assets | 4,997 | 0
Other liabilities | 0 | 1,693
Level 2 | Estimated Fair Value | Commercial loan hedges |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Other liabilities | 36 | 174
Level 2 | Agency pass-through certificates | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Held-to-maturity securities | 1,750,886 | 1,646,856
Level 2 | Agency pass-through certificates | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Held-to-maturity securities | 1,738,337 | 1,635,913
Level 2 | U.S. government and agency securities | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 194,869 | 211,077
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 | 194,869 | 211,077
Level 2 | Municipal bonds | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 26,212 | 26,624
Level 2 | Municipal bonds | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 26,212 | 26,624
Level 2 | Corporate debt securities | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 184,416 | 185,298
Level 2 | Corporate debt securities | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 184,416 | 185,298
Level 2 | Agency pass-through certificates | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 831,489 | 834,297
Level 2 | Agency pass-through certificates | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 831,489 | 834,297
Level 2 | Commercial MBS | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 8,366 | 8,391
Held-to-maturity securities | 15,000 | 0
Level 2 | Commercial MBS | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Available-for-sale securities | 8,366 | 8,391
Held-to-maturity securities | 15,005 | 0
Level 3 | Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable | 11,107,042 | 10,882,622
Level 3 | Estimated Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Loans receivable | $ 11,503,407 | $ 11,247,586

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: 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=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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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 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. + 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/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: 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=77997519&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 + 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://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=75031641&loc=d3e63345-112809 + Details
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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=82897076&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 + 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -URI http://asc.fasb.org/subtopic&trid=2196772 + Details
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