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INVESTMENT SECURITIES - Amortized cost and estimated fair value of investment securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Available-for-Sale Portfolio:
Amortized Cost	$ 3,157,610	$ 3,192,392
Gross Unrealized Gains	383	325
Gross Unrealized Losses	298,681	407,003
Fair Value	2,859,312	2,785,714
Held-to-Maturity Portfolio:
Amortized Cost	1,442,308	1,345,155
Gross Unrealized Gains	2,502	448
Gross Unrealized Losses	83,921	125,435
Fair Value	1,360,889	1,220,168
Other equity securities, at adjusted cost
Equity securities, Fair values	2,900	2,400
Equity securities without readily determinable fair value amount	76,000	85,100
U.S. Treasury and agency securities
Available-for-Sale Portfolio:
Amortized Cost	2,396,816	2,389,208
Gross Unrealized Gains	0	0
Gross Unrealized Losses	218,906	305,422
Fair Value	2,177,910	2,083,786
Held-to-Maturity Portfolio:
Amortized Cost	1,160,978	1,138,743
Gross Unrealized Gains	0	0
Gross Unrealized Losses	83,632	123,194
Fair Value	1,077,346	1,015,549
Obligations of state and political subdivisions
Available-for-Sale Portfolio:
Amortized Cost	418,563	428,204
Gross Unrealized Gains	298	288
Gross Unrealized Losses	33,420	41,997
Fair Value	385,441	386,495
Government agency mortgage-backed securities
Available-for-Sale Portfolio:
Amortized Cost	332,181	360,102
Gross Unrealized Gains	85	37
Gross Unrealized Losses	46,017	58,915
Fair Value	286,249	301,224
Held-to-Maturity Portfolio:
Amortized Cost	281,330	206,412
Gross Unrealized Gains	2,502	448
Gross Unrealized Losses	289	2,241
Fair Value	283,543	204,619
Corporate debt securities
Available-for-Sale Portfolio:
Amortized Cost	5,000	8,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	125	303
Fair Value	4,875	7,697
Government agency collateralized mortgage obligations
Available-for-Sale Portfolio:
Amortized Cost	5,050	6,878
Gross Unrealized Gains	0	0
Gross Unrealized Losses	213	366
Fair Value	4,837	6,512
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	36,900	45,400
Federal Reserve Bank Common Stock
Other equity securities, at adjusted cost
Equity securities without readily determinable fair value amount	33,300	33,400
Other Equity Securities
Other equity securities, at adjusted cost
Equity securities without readily determinable fair value amount	$ 5,800	$ 6,300

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