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INVESTMENT SECURITIES - Amortized cost and estimated fair value of investment securities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Available-for-Sale Portfolio:
Amortized Cost	$ 3,241,610	$ 3,301,607
Gross Unrealized Gains	353	1,545
Gross Unrealized Losses	433,010	383,160
Fair Value	2,808,953	2,919,992
Held-to-Maturity Portfolio:
Amortized Cost	1,271,109	1,172,174
Gross Unrealized Gains	343	688
Gross Unrealized Losses	103,697	51,046
Fair Value	1,167,755	1,121,816
Equity and other Securities:
Equity and other securities	86,500	73,146
Other equity securities, at adjusted cost
Equity securities, Fair values		400
Equity securities without readily determinable fair value amount	84,500	72,800
U.S. Treasury and agency securities
Available-for-Sale Portfolio:
Amortized Cost	2,385,193	2,381,168
Gross Unrealized Gains	0	0
Gross Unrealized Losses	329,145	300,385
Fair Value	2,056,048	2,080,783
Held-to-Maturity Portfolio:
Amortized Cost	1,123,825	1,109,101
Gross Unrealized Gains	0	0
Gross Unrealized Losses	102,042	50,866
Fair Value	1,021,783	1,058,235
Obligations of state and political subdivisions
Available-for-Sale Portfolio:
Amortized Cost	459,849	502,879
Gross Unrealized Gains	330	1,469
Gross Unrealized Losses	42,736	29,985
Fair Value	417,443	474,363
Government agency mortgage-backed securities
Available-for-Sale Portfolio:
Amortized Cost	380,373	400,062
Gross Unrealized Gains	23	76
Gross Unrealized Losses	60,101	51,612
Fair Value	320,295	348,526
Held-to-Maturity Portfolio:
Amortized Cost	147,284	63,073
Gross Unrealized Gains	343	688
Gross Unrealized Losses	1,655	180
Fair Value	145,972	63,581
Corporate debt securities
Available-for-Sale Portfolio:
Amortized Cost	8,000	8,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	503	606
Fair Value	7,497	7,394
Government agency collateralized mortgage obligations
Available-for-Sale Portfolio:
Amortized Cost	8,195	9,498
Gross Unrealized Gains	0	0
Gross Unrealized Losses	525	572
Fair Value	7,670	8,926
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	44,800	32,500
Federal Reserve Bank Common Stock
Other equity securities, at adjusted cost
Equity securities, Fair values	2,000
Equity securities without readily determinable fair value amount	33,600	33,600
Other Equity Securities
Other equity securities, at adjusted cost
Equity securities without readily determinable fair value amount	$ 6,100	$ 6,700

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