SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended												12 Months Ended
	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	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, 2024 USD ($) segment	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of reporting segments | segment													3
Segment data [Abstract]
Net interest income	$ 115,922	$ 110,859	$ 118,301	$ 110,036	$ 104,934	$ 97,465	$ 101,405	$ 84,057	$ 79,760	$ 72,151	$ 83,800	$ 71,613	$ 455,118	$ 387,861	$ 307,324
Provision for (reversal of) credit loss													42,238	57,448	28,862
Noninterest income	52,010	$ 65,871	$ 128,945	$ 52,761	56,051	$ 67,733	$ 127,038	$ 65,777	43,456	$ 53,994	$ 109,766	$ 86,591	299,587	316,599	293,807
Noninterest expense													513,253	464,975	385,275
Income before income tax expense													198,791	182,131	187,318
Total assets	7,549,336				7,535,543				6,747,410				7,549,336	7,535,543	6,747,410
Total goodwill	309,505				309,505				309,505				309,505	309,505	309,505
Total deposits	5,875,085				6,589,182				5,866,037				5,875,085	6,589,182	5,866,037
Total Committed Loans
Segment data [Abstract]
Provision for (reversal of) credit loss													42,661	57,354	28,538
Consumer
Segment data [Abstract]
Net interest income													236,011	154,316	98,366
Noninterest income													217,107	233,544	189,252
Noninterest expense													207,778	165,782	99,589
Income before income tax expense													218,390	183,158	157,349
Total assets	444,272				492,964				356,994				444,272	492,964	356,994
Total goodwill	87,145				87,145				87,145				87,145	87,145	87,145
Total deposits	5,643,228				6,376,467				5,695,776				5,643,228	6,376,467	5,695,776
Consumer | Total Committed Loans
Segment data [Abstract]
Provision for (reversal of) credit loss													26,950	38,920	30,680
Commercial
Segment data [Abstract]
Net interest income													194,967	195,239	187,209
Noninterest income													71,748	66,051	68,412
Noninterest expense													142,143	141,627	128,904
Income before income tax expense													109,001	101,279	112,043
Total assets	4,437,844				4,179,914				3,487,461				4,437,844	4,179,914	3,487,461
Total goodwill	222,360				222,360				222,360				222,360	222,360	222,360
Total deposits	10,935				5,958				8,965				10,935	5,958	8,965
Commercial | Total Committed Loans
Segment data [Abstract]
Provision for (reversal of) credit loss													15,571	18,384	14,674
Corporate Services/Other
Segment data [Abstract]
Net interest income													24,140	38,306	21,749
Noninterest income													10,732	17,004	36,143
Noninterest expense													163,332	157,566	156,782
Income before income tax expense													(128,600)	(102,306)	(82,074)
Total assets	2,667,220				2,862,665				2,902,955				2,667,220	2,862,665	2,902,955
Total goodwill	0				0				0				0	0	0
Total deposits	$ 220,922				$ 206,757				$ 161,296				220,922	206,757	161,296
Corporate Services/Other | Total Committed Loans
Segment data [Abstract]
Provision for (reversal of) credit loss													$ 140	$ 50	$ (16,816)