FAIR VALUE MEASUREMENTS - Assets Measured at Fair Value on Recurring and Non-recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Available-for-sale Securities [Abstract]
Total debt securities	$ 600,112	$ 586,454
Total available for sale securities	1,992,352	1,693,431
Held-to-maturity Securities [Abstract]
Total held to maturity securities	240,144	564,185
Level 1 [Member]
Available-for-sale Securities [Abstract]
Total available for sale securities	1,829	1,445
Held-to-maturity Securities [Abstract]
Total held to maturity securities	0	0
Level 2 [Member]
Available-for-sale Securities [Abstract]
Total available for sale securities	1,990,523	1,691,986
Held-to-maturity Securities [Abstract]
Total held to maturity securities	240,144	564,185
Level 3 [Member]
Available-for-sale Securities [Abstract]
Total available for sale securities	0	0
Held-to-maturity Securities [Abstract]
Total held to maturity securities	0	0
Recurring [Member]
Available-for-sale Securities [Abstract]
Small business administration securities	56,948	57,871
Obligations of states and political subdivisions	14,610	0
Non-bank qualified obligations of states and political subdivisions	1,223,030	950,829
Asset-baked securities	95,823	96,832
Mortgage-backed securities	600,112	586,454
Total debt securities	1,990,523	1,691,986
Common equities and mutual funds	1,829	1,445
Total available for sale securities	1,992,352	1,693,431
Held-to-maturity Securities [Abstract]
Small business administration securities	0	0
Obligations of states and political subdivisions	4,342	19,368
Non-bank qualified obligations of states and political subdivisions	227,482	432,361
Asset-backed securities	0	0
Mortgage-backed securities	8,320	112,456
Total debt securities	240,144	564,185
Common equities and mutual funds	0	0
Total held to maturity securities	240,144	564,185
Recurring [Member] \| Level 1 [Member]
Available-for-sale 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	1,829	1,445
Total available for sale securities	1,829	1,445
Held-to-maturity 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-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 Securities [Abstract]
Small business administration securities	56,948	57,871
Obligations of states and political subdivisions	14,610	0
Non-bank qualified obligations of states and political subdivisions	1,223,030	950,829
Asset-baked securities	95,823	96,832
Mortgage-backed securities	600,112	586,454
Total debt securities	1,990,523	1,691,986
Common equities and mutual funds	0	0
Total available for sale securities	1,990,523	1,691,986
Held-to-maturity Securities [Abstract]
Small business administration securities	0	0
Obligations of states and political subdivisions	4,342	19,368
Non-bank qualified obligations of states and political subdivisions	227,482	432,361
Asset-backed securities	0	0
Mortgage-backed securities	8,320	112,456
Total debt securities	240,144	564,185
Common equities and mutual funds	0	0
Total held to maturity securities	240,144	564,185
Recurring [Member] \| Level 3 [Member]
Available-for-sale 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
Held-to-maturity 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-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.

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

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Fair value of investments in debt securities classified as held-to-maturity.

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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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