SECURITIES - Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Available-for-sale debt securities [Abstract]
Amortized Cost	$ 1,201,561	$ 1,244,384
Fair Value	1,228,124	1,268,102
Amortized Cost
Due in one year or less	720	1,385
Due after one year through five years	22,799	20,805
Due after five years through ten years	32,099	32,441
Due after ten years	727,553	749,874
Total	783,171	804,505
Mortgage-backed securities	418,390	439,879
Fair Value
Due in one year or less	724	1,398
Due after one year through five years	23,707	21,769
Due after five years through ten years	33,738	34,025
Due after ten years	739,194	757,303
Total	797,363	814,495
Mortgage-backed securities	430,761	453,607
SBA securities
Available-for-sale debt securities [Abstract]
Amortized Cost	156,929	159,722
Gross Unrealized Gains	5,255	5,391
Gross Unrealized (Losses)	(175)	(158)
Fair Value	162,009	164,955
LESS THAN 12 MONTHS, Fair Value	40,938	32,257
LESS THAN 12 MONTHS, Unrealized (Losses)	(81)	(102)
OVER 12 MONTHS, Fair Value	18,623	9,875
OVER 12 MONTHS, Unrealized (Losses)	(94)	(56)
TOTAL, Fair Value	59,561	42,132
TOTAL, Unrealized (Losses)	(175)	(158)
Obligations of states and political subdivisions
Available-for-sale debt securities [Abstract]
Amortized Cost	2,823	825
Gross Unrealized Gains	17	16
Gross Unrealized (Losses)	0	0
Fair Value	2,840	841
Non-bank qualified obligations of states and political subdivisions
Available-for-sale debt securities [Abstract]
Amortized Cost	299,830	314,819
Gross Unrealized Gains	9,808	8,978
Gross Unrealized (Losses)	0	(23)
Fair Value	309,638	323,774
LESS THAN 12 MONTHS, Fair Value		6,265
LESS THAN 12 MONTHS, Unrealized (Losses)		(6)
OVER 12 MONTHS, Fair Value		3,103
OVER 12 MONTHS, Unrealized (Losses)		(17)
TOTAL, Fair Value		9,368
TOTAL, Unrealized (Losses)		(23)
Asset-backed securities
Available-for-sale debt securities [Abstract]
Amortized Cost	323,589	329,139
Gross Unrealized Gains	2,752	2,015
Gross Unrealized (Losses)	(3,465)	(6,229)
Fair Value	322,876	324,925
LESS THAN 12 MONTHS, Fair Value	60,713	106,474
LESS THAN 12 MONTHS, Unrealized (Losses)	(136)	(1,089)
OVER 12 MONTHS, Fair Value	200,240	178,686
OVER 12 MONTHS, Unrealized (Losses)	(3,329)	(5,140)
TOTAL, Fair Value	260,953	285,160
TOTAL, Unrealized (Losses)	(3,465)	(6,229)
Mortgage-backed securities
Available-for-sale debt securities [Abstract]
Amortized Cost	418,390	439,879
Gross Unrealized Gains	12,685	14,567
Gross Unrealized (Losses)	(314)	(839)
Fair Value	430,761	453,607
LESS THAN 12 MONTHS, Fair Value	62,494	138,338
LESS THAN 12 MONTHS, Unrealized (Losses)	(314)	(839)
OVER 12 MONTHS, Fair Value	0	0
OVER 12 MONTHS, Unrealized (Losses)	0	0
TOTAL, Fair Value	62,494	138,338
TOTAL, Unrealized (Losses)	(314)	(839)
Total debt securities AFS
Available-for-sale debt securities [Abstract]
Amortized Cost	1,201,561	1,244,384
Gross Unrealized Gains	30,517	30,967
Gross Unrealized (Losses)	(3,954)	(7,249)
Fair Value	1,228,124	1,268,102
LESS THAN 12 MONTHS, Fair Value	164,145	283,334
LESS THAN 12 MONTHS, Unrealized (Losses)	(531)	(2,036)
OVER 12 MONTHS, Fair Value	218,863	191,664
OVER 12 MONTHS, Unrealized (Losses)	(3,423)	(5,213)
TOTAL, Fair Value	383,008	474,998
TOTAL, Unrealized (Losses)	$ (3,954)	$ (7,249)

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).