FAIR VALUE OF FINANCIAL INSTRUMENTS - Assets Measured at Fair Value on Recurring and Non-recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Sep. 30, 2020
Available-for-sale debt securities [Abstract]
Mortgage-backed securities	$ 1,063,582	$ 453,607
Level 1
Available-for-sale debt securities [Abstract]
Debt Securities, Available-for-sale, Total	0	0
Common equities and mutual funds	3,324	2,969
Level 2
Available-for-sale debt securities [Abstract]
Debt Securities, Available-for-sale, Total	1,917,605	1,268,102
Common equities and mutual funds	0	0
Level 3
Available-for-sale debt securities [Abstract]
Debt Securities, Available-for-sale, Total	0	0
Common equities and mutual funds	0	0
Recurring
Available-for-sale debt securities [Abstract]
Small business administration securities	169,185	164,955
Obligations of states and political subdivisions	2,766	841
Non-bank qualified obligations of states and political subdivisions	274,350	323,774
Asset-baked securities	407,722	324,925
Mortgage-backed securities	1,063,582	453,607
Debt Securities, Available-for-sale, Total	1,917,605	1,268,102
Common equities and mutual funds	3,324	2,969
Non-marketable equity securities	4,724	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
Debt Securities, Available-for-sale, Total	0	0
Common equities and mutual funds	3,324	2,969
Non-marketable equity securities	0	0
Recurring \| Level 2
Available-for-sale debt securities [Abstract]
Small business administration securities	169,185	164,955
Obligations of states and political subdivisions	2,766	841
Non-bank qualified obligations of states and political subdivisions	274,350	323,774
Asset-baked securities	407,722	324,925
Mortgage-backed securities	1,063,582	453,607
Debt Securities, Available-for-sale, Total	1,917,605	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
Debt Securities, Available-for-sale, Total	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.

+ Details

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Balance Type:	debit
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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.

+ Details

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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.

+ Details

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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=124258926&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=124260329&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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Balance Type:	debit
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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), classified as current.

+ 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=123594786&loc=SL75136599-209740

+ Details

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Period Type:	instant

- 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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