FAIR VALUE MEASUREMENTS - Assets Measured at Fair Value on Recurring and Non-recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Available-for-sale debt securities [Abstract]
Total debt securities	$ 575,999	$ 586,454
Total available for sale securities	1,927,537	1,693,431
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Total held to maturity securities	214,158	564,185
Level 1 [Member]
Available-for-sale debt securities [Abstract]
Total available for sale securities	1,896	1,445
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Total held to maturity securities	0	0
Level 2 [Member]
Available-for-sale debt securities [Abstract]
Total available for sale securities	1,925,641	1,691,986
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Total held to maturity securities	214,158	564,185
Level 3 [Member]
Available-for-sale debt securities [Abstract]
Total available for sale securities	0	0
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Total held to maturity securities	0	0
Recurring [Member]
Available-for-sale debt securities [Abstract]
Small business administration securities	50,959	57,871
Obligations of states and political subdivisions	14,370	0
Non-bank qualified obligations of states and political subdivisions	1,083,474	950,829
Asset-baked securities	200,839	96,832
Mortgage-backed securities	575,999	586,454
Total debt securities	1,925,641	1,691,986
Common equities and mutual funds	1,896	1,445
Total available for sale securities	1,927,537	1,693,431
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Small business administration securities	0	0
Obligations of states and political subdivisions	3,826	19,368
Non-bank qualified obligations of states and political subdivisions	202,486	432,361
Asset-backed securities	0	0
Mortgage-backed securities	7,846	112,456
Total debt securities	214,158	564,185
Common equities and mutual funds	0	0
Total held to maturity securities	214,158	564,185
Recurring [Member] \| Level 1 [Member]
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
Total debt securities	0	0
Common equities and mutual funds	1,896	1,445
Total available for sale securities	1,896	1,445
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [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-backed securities	0	0
Mortgage-backed securities	0	0
Total debt securities	0	0
Common equities and mutual funds	0	0
Total held to maturity securities	0	0
Recurring [Member] \| Level 2 [Member]
Available-for-sale debt securities [Abstract]
Small business administration securities	50,959	57,871
Obligations of states and political subdivisions	14,370	0
Non-bank qualified obligations of states and political subdivisions	1,083,474	950,829
Asset-baked securities	200,839	96,832
Mortgage-backed securities	575,999	586,454
Total debt securities	1,925,641	1,691,986
Common equities and mutual funds	0	0
Total available for sale securities	1,925,641	1,691,986
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Small business administration securities	0	0
Obligations of states and political subdivisions	3,826	19,368
Non-bank qualified obligations of states and political subdivisions	202,486	432,361
Asset-backed securities	0	0
Mortgage-backed securities	7,846	112,456
Total debt securities	214,158	564,185
Common equities and mutual funds	0	0
Total held to maturity securities	214,158	564,185
Recurring [Member] \| Level 3 [Member]
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
Total debt securities	0	0
Common equities and mutual funds	0	0
Total available for sale securities	0	0
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [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-backed securities	0	0
Mortgage-backed securities	0	0
Total debt securities	0	0
Common equities and mutual funds	0	0
Total held to maturity securities	$ 0	$ 0

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

No definition available.

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Fair value of asset backed securities investments classified as held To maturity.

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Fair value of debt securities categorized as held-to-maturity.

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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.

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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.

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No definition available.

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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 held-to-maturity.

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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 held-to-maturity.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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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 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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

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Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as held-to-maturity.

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