INVESTMENT SECURITIES - Unrealized Losses and Fair Value of Securities (Details) $ in Thousands	Sep. 30, 2025 USD ($) security	Sep. 30, 2024 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss | security	213	209
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (1,446)	$ (429)
12 months or more, unrealized gross losses	(48,788)	(54,270)
Total, unrealized gross losses	(50,234)	(54,699)
Less than 12 months, fair value	279,282	239,441
12 months or more, fair value	941,676	1,125,962
Total, fair value	1,220,958	1,365,403
Held-to-maturity securities
Less than 12 months, unrealized gross losses	0	0
12 months or more, unrealized gross losses	(37,136)	(36,839)
Total unrealized gross losses	(37,136)	(36,839)
Less than 12 months, fair value	0	0
12 months or more, fair value	310,597	348,573
Total, fair value	310,597	348,573
Less than 12 months, unrealized gross losses	(1,446)	(429)
12 months or more, unrealized gross losses	(85,924)	(91,109)
Total unrealized gross losses	(87,370)	(91,538)
Less than 12 months, fair value	279,282	239,441
12 months or more, fair value	1,252,273	1,474,535
Total fair value	1,531,555	1,713,976
Corporate debt securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(119)	0
12 months or more, unrealized gross losses	(8,495)	(10,993)
Total, unrealized gross losses	(8,614)	(10,993)
Less than 12 months, fair value	19,881	0
12 months or more, fair value	102,342	146,060
Total, fair value	122,223	146,060
Municipal bonds
Available-for-sale securities
Less than 12 months, unrealized gross losses	0	(15)
12 months or more, unrealized gross losses	(405)	(394)
Total, unrealized gross losses	(405)	(409)
Less than 12 months, fair value	0	19,985
12 months or more, fair value	15,008	15,088
Total, fair value	15,008	35,073
U.S. government and agency securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(483)
12 months or more, unrealized gross losses	(127)
Total, unrealized gross losses	(610)
Less than 12 months, fair value	143,444
12 months or more, fair value	35,211
Total, fair value	178,655
Asset-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(62)	(249)
12 months or more, unrealized gross losses	(1,424)	(2,373)
Total, unrealized gross losses	(1,486)	(2,622)
Less than 12 months, fair value	34,932	116,173
12 months or more, fair value	135,315	235,846
Total, fair value	170,247	352,019
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(782)	(165)
12 months or more, unrealized gross losses	(38,337)	(40,510)
Total, unrealized gross losses	(39,119)	(40,675)
Less than 12 months, fair value	81,025	103,283
12 months or more, fair value	653,800	728,968
Total, fair value	$ 734,825	$ 832,251