SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended												12 Months Ended
	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment													3
Segment data [Abstract]
Net interest income (expense)	$ 104,934	$ 97,465	$ 101,405	$ 84,057	$ 79,760	$ 72,151	$ 83,800	$ 71,613	$ 70,667	$ 68,475	$ 73,850	$ 65,999	$ 387,861	$ 307,324	$ 278,991
Provision for (reversal of) credit losses	9,042	1,773	36,763	9,776	(2,648)	(1,302)	32,302	186	8,775	4,612	30,290	6,089	57,354	28,538	49,766
Noninterest income	56,051	$ 67,733	$ 127,038	$ 65,777	43,456	$ 53,994	$ 109,766	$ 86,591	49,542	$ 62,453	$ 113,453	$ 45,455	316,599	293,807	270,904
Noninterest expense													464,975	385,275	343,683
Income before income tax expense													182,131	187,318	156,446
Total assets	7,535,543				6,747,410				6,690,650				7,535,543	6,747,410	6,690,650
Goodwill	309,505				309,505				309,505				309,505	309,505	309,505
Total deposits	6,589,182				5,866,037				5,514,971				6,589,182	5,866,037	5,514,971
Consumer
Segment data [Abstract]
Net interest income (expense)													154,316	98,366
Noninterest income													233,544	189,252
Consumer | Operating Segments
Segment data [Abstract]
Net interest income (expense)													154,316	98,366	91,489
Provision for (reversal of) credit losses													38,920	30,680	35,765
Noninterest income													233,544	189,252	195,708
Noninterest expense													165,782	99,589	90,792
Income before income tax expense													183,158	157,349	160,640
Total assets	492,964				356,994				354,441				492,964	356,994	354,441
Goodwill	87,145				87,145				87,145				87,145	87,145	87,145
Total deposits	6,376,467				5,695,776				5,342,192				6,376,467	5,695,776	5,342,192
Commercial
Segment data [Abstract]
Net interest income (expense)													195,239	187,209
Noninterest income													66,051	68,412
Commercial | Operating Segments
Segment data [Abstract]
Net interest income (expense)													195,239	187,209	173,969
Provision for (reversal of) credit losses													18,384	14,674	19,791
Noninterest income													66,051	68,412	61,813
Noninterest expense													141,627	128,904	114,925
Income before income tax expense													101,279	112,043	101,066
Total assets	4,179,914				3,487,461				3,208,889				4,179,914	3,487,461	3,208,889
Goodwill	222,360				222,360				222,360				222,360	222,360	222,360
Total deposits	5,958				8,965				6,625				5,958	8,965	6,625
Corporate Services/Other
Segment data [Abstract]
Net interest income (expense)													38,306	21,749
Noninterest income													17,004	36,143
Corporate Services/Other | Operating Segments
Segment data [Abstract]
Net interest income (expense)													38,306	21,749	13,533
Provision for (reversal of) credit losses													50	(16,816)	(5,790)
Noninterest income													17,004	36,143	13,383
Noninterest expense													157,566	156,782	137,966
Income before income tax expense													(102,306)	(82,074)	(105,260)
Total assets	2,862,665				2,902,955				3,127,320				2,862,665	2,902,955	3,127,320
Goodwill	0				0				0				0	0	0
Total deposits	$ 206,757				$ 161,296				$ 166,154				$ 206,757	$ 161,296	$ 166,154