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Fair Value Measurements - Additional Information (Details) - USD ($) | 9 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jan. 01, 2020 | Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Held-to-maturity securities, at amortized cost | $ 827,316,000 |  |  | $ 1,443,480,000
Available-for-sale, at amortized cost | 2,022,414,000 |  |  | 1,458,071,000
Available-for-sale securities purchased | 684,292,000 | $ 327,670,000 |  |
Proceeds from sales of available-for-sale securities | 204,351,000 | 491,000 |  |
Held-to-maturity securities purchased | 0 | 0 |  |
Proceeds from sale of held-to-maturity securities | $ 0 | $ 0 |  |
Term of contractual due dates of substantially all mortgage-backed securities (in years) | 10 years |  |  |
Accrued interest for available-for-sale securities | $ 4,154,000 |  |  | 3,712,000
Accrued interest for held-to-maturity securities | 2,126,000 |  |  | $ 3,716,000
Mortgage-backed securities | Accounting Standards Update 2019-04 |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Held-to-maturity securities, at amortized cost |  |  | $ (374,680,000) |
Available-for-sale, at amortized cost |  |  | $ 374,680,000 |
Consumer loans | Single-family residential |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Foreclosed residential real estate properties held as REO | 1,346,000 |  |  |
Real estate acquired through foreclosure | $ 4,104,000 |  |  |

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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 + Details
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- Definition Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10(4)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
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- Definition Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5162-111524 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4437-111522 + Details
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