INVESTMENT SECURITIES - Amortized cost and estimated fair value of investment securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Available-for-Sale Portfolio:
Amortized Cost	$ 3,301,607	$ 4,675,474
Gross Unrealized Gains	1,545	565
Gross Unrealized Losses	383,160	524,188
Fair Value	2,919,992	4,151,851
Held-to-Maturity Portfolio:
Amortized Cost	1,172,174	1,079,695
Gross Unrealized Gains	688	0
Gross Unrealized Losses	51,046	44,900
Fair Value	1,121,816	1,034,795
Equity securities, at fair value
Amortized Cost	251	251
Gross Unrealized Gains	121	168
Gross Unrealized Losses	0	0
Fair Value	372	419
Equity and other Securities:
Amortized cost	72,275	82,424
Gross Unrealized Gains	871	918
Gross Unrealized Losses	0	0
Equity and other securities	73,146	83,342
U.S. Treasury and agency securities
Available-for-Sale Portfolio:
Amortized Cost	2,381,168	3,660,546
Gross Unrealized Gains	0	0
Gross Unrealized Losses	300,385	417,009
Fair Value	2,080,783	3,243,537
Held-to-Maturity Portfolio:
Amortized Cost	1,109,101	1,079,695
Gross Unrealized Gains	0	0
Gross Unrealized Losses	50,866	44,900
Fair Value	1,058,235	1,034,795
Obligations of state and political subdivisions
Available-for-Sale Portfolio:
Amortized Cost	502,879	549,118
Gross Unrealized Gains	1,469	506
Gross Unrealized Losses	29,985	45,327
Fair Value	474,363	504,297
Government agency mortgage-backed securities
Available-for-Sale Portfolio:
Amortized Cost	400,062	444,689
Gross Unrealized Gains	76	58
Gross Unrealized Losses	51,612	60,114
Fair Value	348,526	384,633
Held-to-Maturity Portfolio:
Amortized Cost	63,073	0
Gross Unrealized Gains	688	0
Gross Unrealized Losses	180	0
Fair Value	63,581	0
Corporate debt securities
Available-for-Sale Portfolio:
Amortized Cost	8,000	8,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	606	886
Fair Value	7,394	7,114
Government agency collateralized mortgage obligations
Available-for-Sale Portfolio:
Amortized Cost	9,498	13,121
Gross Unrealized Gains	0	1
Gross Unrealized Losses	572	852
Fair Value	8,926	12,270
Held-to-Maturity Portfolio:
Amortized Cost	0
Fair Value	0
Federal Home Loan Bank Common Stock
Other equity securities, at adjusted cost
Amortized Cost	32,526	47,497
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	32,526	47,497
Federal Reserve Bank Common Stock
Other equity securities, at adjusted cost
Amortized Cost	33,568	31,144
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	33,568	31,144
Other Equity Securities
Other equity securities, at adjusted cost
Amortized Cost	5,930	3,532
Gross Unrealized Gains	750	750
Gross Unrealized Losses	0	0
Fair Value	$ 6,680	$ 4,282

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