SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Quarterly Financial Information Disclosure [Abstract]
Interest and dividend income	$ 119,750	$ 113,942	$ 126,761	$ 115,898	$ 109,642	$ 99,346	$ 104,687	$ 85,060	$ 80,222	$ 73,906	$ 85,177	$ 72,891	$ 476,351	$ 398,735	$ 312,196
Interest expense	3,828	3,083	8,460	5,862	4,708	1,881	3,282	1,003	462	1,755	1,377	1,278	21,233	10,874	4,872
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	838	5,881	26,052	9,890	9,042	1,773	36,763	9,776	(2,648)	(1,302)	32,302	186	42,661	57,354	28,538
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
Net income attributable to parent	$ 33,597	$ 41,835	$ 65,268	$ 27,657	$ 35,906	$ 45,096	$ 54,771	$ 27,842	$ 23,420	$ 22,391	$ 49,251	$ 61,324	$ 168,357	$ 163,615	$ 156,386
Earnings per common share
Basic (in dollars per share)	$ 1.35	$ 1.66	$ 2.56	$ 1.06	$ 1.37	$ 1.69	$ 1.99	$ 0.98	$ 0.81	$ 0.76	$ 1.66	$ 2.00	$ 6.63	$ 6.01	$ 5.26
Diluted (in dollars per share)	1.35	1.66	2.56	1.06	1.36	1.68	1.99	0.98	0.81	0.76	1.66	2.00	6.62	5.99	5.26
Dividend declared per share (in dollars per share)	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ 0.20	$ 0.20	$ 0.20