INVESTMENT SECURITIES - Amortized cost and estimated fair value of investment securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2024	Dec. 31, 2023
Available-for-Sale Portfolio:
Amortized Cost	$ 3,287,497	$ 3,301,607
Gross Unrealized Gains	676	1,545
Gross Unrealized Losses	424,309	383,160
Fair Value	2,863,864	2,919,992
Held-to-Maturity Portfolio:
Amortized Cost	1,217,472	1,172,174
Gross Unrealized Gains	477	688
Gross Unrealized Losses	86,334	51,046
Fair Value	1,131,615	1,121,816
Equity securities, at fair value
Amortized Cost	252	251
Gross Unrealized Gains	136	121
Gross Unrealized Losses	0	0
Fair Value	388	372
Equity and other Securities:
Amortized cost	69,892	72,275
Gross Unrealized Gains	886	871
Gross Unrealized Losses	0	0
Equity and other securities	70,778	73,146
U.S. Treasury and agency securities
Available-for-Sale Portfolio:
Amortized Cost	2,382,653	2,381,168
Gross Unrealized Gains	0	0
Gross Unrealized Losses	327,136	300,385
Fair Value	2,055,517	2,080,783
Held-to-Maturity Portfolio:
Amortized Cost	1,116,458	1,109,101
Gross Unrealized Gains	0	0
Gross Unrealized Losses	85,460	50,866
Fair Value	1,030,998	1,058,235
Obligations of state and political subdivisions
Available-for-Sale Portfolio:
Amortized Cost	496,983	502,879
Gross Unrealized Gains	630	1,469
Gross Unrealized Losses	37,464	29,985
Fair Value	460,149	474,363
Government agency mortgage-backed securities
Available-for-Sale Portfolio:
Amortized Cost	391,020	400,062
Gross Unrealized Gains	46	76
Gross Unrealized Losses	58,587	51,612
Fair Value	332,479	348,526
Held-to-Maturity Portfolio:
Amortized Cost	101,014	63,073
Gross Unrealized Gains	477	688
Gross Unrealized Losses	874	180
Fair Value	100,617	63,581
Corporate debt securities
Available-for-Sale Portfolio:
Amortized Cost	8,000	8,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	560	606
Fair Value	7,440	7,394
Government agency collateralized mortgage obligations
Available-for-Sale Portfolio:
Amortized Cost	8,841	9,498
Gross Unrealized Gains	0	0
Gross Unrealized Losses	562	572
Fair Value	8,279	8,926
Held-to-Maturity Portfolio:
Amortized Cost	0
Fair Value	0
Federal Home Loan Bank Common Stock
Other equity securities, at adjusted cost
Amortized Cost	29,866	32,526
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	29,866	32,526
Federal Reserve Bank Common Stock
Other equity securities, at adjusted cost
Amortized Cost	33,568	33,568
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	33,568	33,568
Other Equity Securities
Other equity securities, at adjusted cost
Amortized Cost	6,206	5,930
Gross Unrealized Gains	750	750
Gross Unrealized Losses	0	0
Fair Value	$ 6,956	$ 6,680

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