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INVESTMENT SECURITIES - Amortized cost and estimated fair value of investment securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Available-for-Sale Portfolio:
Amortized Cost	$ 3,192,392	$ 3,301,607
Gross Unrealized Gains	325	1,545
Gross Unrealized Losses	407,003	383,160
Held-to-Maturity Portfolio:
Amortized Cost	1,345,155	1,172,174
Gross Unrealized Gains	448	688
Gross Unrealized Losses	125,435	51,046
Fair Value	1,220,168	1,121,816
Other equity securities, at adjusted cost
Equity securities, Fair values		400
Equity securities without readily determinable fair value amount	85,100	72,800
U.S. Treasury and agency securities
Available-for-Sale Portfolio:
Amortized Cost	2,389,208	2,381,168
Gross Unrealized Gains	0	0
Gross Unrealized Losses	305,422	300,385
Held-to-Maturity Portfolio:
Amortized Cost	1,138,743	1,109,101
Gross Unrealized Gains	0	0
Gross Unrealized Losses	123,194	50,866
Fair Value	1,015,549	1,058,235
Obligations of state and political subdivisions
Available-for-Sale Portfolio:
Amortized Cost	428,204	502,879
Gross Unrealized Gains	288	1,469
Gross Unrealized Losses	41,997	29,985
Government agency mortgage-backed securities
Available-for-Sale Portfolio:
Amortized Cost	360,102	400,062
Gross Unrealized Gains	37	76
Gross Unrealized Losses	58,915	51,612
Held-to-Maturity Portfolio:
Amortized Cost	206,412	63,073
Gross Unrealized Gains	448	688
Gross Unrealized Losses	2,241	180
Fair Value	204,619	63,581
Corporate debt securities
Available-for-Sale Portfolio:
Amortized Cost	8,000	8,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	303	606
Government agency collateralized mortgage obligations
Available-for-Sale Portfolio:
Amortized Cost	6,878	9,498
Gross Unrealized Gains	0	0
Gross Unrealized Losses	366	572
Held-to-Maturity Portfolio:
Amortized Cost	0
Fair Value	0
Federal Home Loan Bank Common Stock
Other equity securities, at adjusted cost
Equity securities without readily determinable fair value amount	45,400	32,500
Federal Reserve Bank Common Stock
Other equity securities, at adjusted cost
Equity securities, Fair values	2,400
Equity securities without readily determinable fair value amount	33,400	33,600
Other Equity Securities
Other equity securities, at adjusted cost
Equity securities without readily determinable fair value amount	$ 6,300	$ 6,700

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