FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets Measured at Fair Value on Recurring and Non-recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Available-for-sale debt securities [Abstract]
Mortgage-backed securities	$ 558,833	$ 453,607
Fair Value	1,480,780	1,268,102
Level 1
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Available-for-sale debt securities [Abstract]
Fair Value	0	0
Common equities and mutual funds	3,235	2,969
Level 2
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Available-for-sale debt securities [Abstract]
Fair Value	1,480,780	1,268,102
Common equities and mutual funds	0	0
Level 3
Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	3,655,832	3,307,037
Available-for-sale debt securities [Abstract]
Fair Value	0	0
Common equities and mutual funds	0	0
Recurring
Available-for-sale debt securities [Abstract]
Small business administration securities	170,006	164,955
Obligations of states and political subdivisions	2,816	841
Non-bank qualified obligations of states and political subdivisions	295,722	323,774
Asset-baked securities	453,403	324,925
Mortgage-backed securities	558,833	453,607
Fair Value	1,480,780	1,268,102
Common equities and mutual funds	3,235	2,969
Non-marketable equity securities	4,265	2,784
Recurring \| Level 1
Available-for-sale debt securities [Abstract]
Small business administration securities	0	0
Obligations of states and political subdivisions	0	0
Non-bank qualified obligations of states and political subdivisions	0	0
Asset-baked securities	0	0
Mortgage-backed securities	0	0
Fair Value	0	0
Common equities and mutual funds	3,235	2,969
Non-marketable equity securities	0	0
Recurring \| Level 2
Available-for-sale debt securities [Abstract]
Small business administration securities	170,006	164,955
Obligations of states and political subdivisions	2,816	841
Non-bank qualified obligations of states and political subdivisions	295,722	323,774
Asset-baked securities	453,403	324,925
Mortgage-backed securities	558,833	453,607
Fair Value	1,480,780	1,268,102
Common equities and mutual funds	0	0
Non-marketable equity securities	0	0
Recurring \| Level 3
Available-for-sale debt securities [Abstract]
Small business administration securities	0	0
Obligations of states and political subdivisions	0	0
Non-bank qualified obligations of states and political subdivisions	0	0
Asset-baked securities	0	0
Mortgage-backed securities	0	0
Fair Value	0	0
Common equities and mutual funds	0	0
Non-marketable equity securities	$ 0	$ 0

Fair value of asset backed securities investments classified as available for sale.

No definition available.

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents common equities and mutual funds as of the balance sheet date which have been categorized as available-for-sale.

+ References

No definition available.

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents non-bank qualified obligations of states and political subdivision as of the balance sheet date which have been categorized as available-for-sale.

+ References

No definition available.

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents obligations of states and political subdivision as of the balance sheet date which have been categorized as available-for-sale.

+ References

No definition available.

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents debt securities issued by the Small Business Administration (SBA) as of the balance sheet date which have been categorized as available-for-sale.

+ References

No definition available.

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- References

No definition available.

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- Definition

Amount 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 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

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- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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- Definition

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.

+ References

No definition available.

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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

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- Definition

Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as available-for-sale.

+ References

No definition available.

+ Details

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