SECURITIES - Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Sep. 30, 2021
Available-for-sale debt securities [Abstract]
Amortized Cost	$ 2,136,832	$ 1,855,084
Fair Value	2,043,478	1,864,899
Amortized Cost
Due in one year or less	240	810
Due after one year through five years	11,029	13,026
Due after five years through ten years	74,314	50,785
Due after ten years	690,508	773,985
Total	776,091	838,606
Mortgage-backed securities	1,360,741	1,016,478
Fair Value
Due in one year or less	241	822
Due after one year through five years	10,855	13,378
Due after five years through ten years	71,056	52,357
Due after ten years	675,208	781,313
Total	757,360	847,870
Mortgage-backed securities	1,286,118	1,017,029
Corporate securities
Available-for-sale debt securities [Abstract]
Amortized Cost	25,000	25,000
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(1,750)	0
Fair Value	23,250	25,000
LESS THAN 12 MONTHS, Fair Value	23,250
LESS THAN 12 MONTHS, Gross Unrealized (Losses)	(1,750)
OVER 12 MONTHS, Fair Value	0
OVER 12 MONTHS, Gross Unrealized (Losses)	0
TOTAL, Fair Value	23,250
TOTAL, Gross Unrealized (Losses)	(1,750)
SBA securities
Available-for-sale debt securities [Abstract]
Amortized Cost	137,330	151,958
Gross Unrealized Gains	1,429	5,251
Gross Unrealized (Losses)	(1,359)	0
Fair Value	137,400	157,209
LESS THAN 12 MONTHS, Fair Value	50,022
LESS THAN 12 MONTHS, Gross Unrealized (Losses)	(1,359)
OVER 12 MONTHS, Fair Value	0
OVER 12 MONTHS, Gross Unrealized (Losses)	0
TOTAL, Fair Value	50,022
TOTAL, Gross Unrealized (Losses)	(1,359)
Obligations of states and political subdivisions
Available-for-sale debt securities [Abstract]
Amortized Cost	2,777	2,497
Gross Unrealized Gains	1	10
Gross Unrealized (Losses)	(72)	0
Fair Value	2,706	2,507
LESS THAN 12 MONTHS, Fair Value	2,158
LESS THAN 12 MONTHS, Gross Unrealized (Losses)	(72)
OVER 12 MONTHS, Fair Value	0
OVER 12 MONTHS, Gross Unrealized (Losses)	0
TOTAL, Fair Value	2,158
TOTAL, Gross Unrealized (Losses)	(72)
Non-bank qualified obligations of states and political subdivisions
Available-for-sale debt securities [Abstract]
Amortized Cost	249,576	266,048
Gross Unrealized Gains	153	3,347
Gross Unrealized (Losses)	(9,306)	(1,100)
Fair Value	240,423	268,295
LESS THAN 12 MONTHS, Fair Value	214,073	101,046
LESS THAN 12 MONTHS, Gross Unrealized (Losses)	(9,180)	(1,100)
OVER 12 MONTHS, Fair Value	1,314	0
OVER 12 MONTHS, Gross Unrealized (Losses)	(126)	0
TOTAL, Fair Value	215,387	101,046
TOTAL, Gross Unrealized (Losses)	(9,306)	(1,100)
Asset-backed securities
Available-for-sale debt securities [Abstract]
Amortized Cost	361,408	393,103
Gross Unrealized Gains	1,636	3,003
Gross Unrealized (Losses)	(9,463)	(1,247)
Fair Value	353,581	394,859
LESS THAN 12 MONTHS, Fair Value	154,196	127,110
LESS THAN 12 MONTHS, Gross Unrealized (Losses)	(4,375)	(283)
OVER 12 MONTHS, Fair Value	111,397	91,553
OVER 12 MONTHS, Gross Unrealized (Losses)	(5,088)	(964)
TOTAL, Fair Value	265,593	218,663
TOTAL, Gross Unrealized (Losses)	(9,463)	(1,247)
Mortgage-backed securities
Available-for-sale debt securities [Abstract]
Amortized Cost	1,360,741	1,016,478
Gross Unrealized Gains	232	9,728
Gross Unrealized (Losses)	(74,855)	(9,177)
Fair Value	1,286,118	1,017,029
LESS THAN 12 MONTHS, Fair Value	1,014,462	759,035
LESS THAN 12 MONTHS, Gross Unrealized (Losses)	(58,435)	(7,418)
OVER 12 MONTHS, Fair Value	206,040	60,792
OVER 12 MONTHS, Gross Unrealized (Losses)	(16,420)	(1,759)
TOTAL, Fair Value	1,220,502	819,827
TOTAL, Gross Unrealized (Losses)	(74,855)	(9,177)
Total debt securities AFS
Available-for-sale debt securities [Abstract]
Amortized Cost	2,136,832	1,855,084
Gross Unrealized Gains	3,451	21,339
Gross Unrealized (Losses)	(96,805)	(11,524)
Fair Value	2,043,478	1,864,899
LESS THAN 12 MONTHS, Fair Value	1,458,161	987,191
LESS THAN 12 MONTHS, Gross Unrealized (Losses)	(75,171)	(8,801)
OVER 12 MONTHS, Fair Value	318,751	152,345
OVER 12 MONTHS, Gross Unrealized (Losses)	(21,634)	(2,723)
TOTAL, Fair Value	1,776,912	1,139,536
TOTAL, Gross Unrealized (Losses)	$ (96,805)	$ (11,524)