FAIR VALUE - Financial assets and liabilities accounted for at fair value on a recurring basis (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Available-for-sale investment securities
Fair Value	$ 2,919,992	$ 4,151,851
Equity securities	372	419
Mortgage loans held for sale		0
Unpaid principal value of mortgage loans held for sale	400
Fair value, assets, levels transfers	0
U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	2,080,783	3,243,537
Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	474,363	504,297
Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	348,526	384,633
Corporate debt securities
Available-for-sale investment securities
Fair Value	7,394	7,114
Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	8,926	12,270
Level 3 \| Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	2	5
Recurring
Available-for-sale investment securities
Fair Value	2,919,992	4,151,851
Equity securities	372	419
Mortgage loans held for sale	414
Total	2,920,786	4,152,280
Recurring \| Interest rate swap agreements
Available-for-sale investment securities
Derivative asset		1
Derivative liability		(1)
Recurring \| Forward sales commitments
Available-for-sale investment securities
Derivative asset	6	5
Recurring \| Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	2	5
Recurring \| U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	2,080,783	3,243,537
Recurring \| Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	474,363	504,297
Recurring \| Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	348,526	384,633
Recurring \| Corporate debt securities
Available-for-sale investment securities
Fair Value	7,394	7,114
Recurring \| Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	8,926	12,270
Recurring \| Level 1
Available-for-sale investment securities
Fair Value	2,019,089	3,178,189
Equity securities	372	419
Mortgage loans held for sale	0
Total	2,019,461	3,178,608
Recurring \| Level 1 \| Interest rate swap agreements
Available-for-sale investment securities
Derivative asset		0
Derivative liability		0
Recurring \| Level 1 \| Forward sales commitments
Available-for-sale investment securities
Derivative asset	0	0
Recurring \| Level 1 \| Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	0	0
Recurring \| Level 1 \| U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	2,019,089	3,178,189
Recurring \| Level 1 \| Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 1 \| Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 1 \| Corporate debt securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 1 \| Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 2
Available-for-sale investment securities
Fair Value	900,903	973,662
Equity securities	0	0
Mortgage loans held for sale	414
Total	901,323	973,667
Recurring \| Level 2 \| Interest rate swap agreements
Available-for-sale investment securities
Derivative asset		1
Derivative liability		(1)
Recurring \| Level 2 \| Forward sales commitments
Available-for-sale investment securities
Derivative asset	6	5
Recurring \| Level 2 \| Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	0	0
Recurring \| Level 2 \| U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	61,694	65,348
Recurring \| Level 2 \| Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	474,363	504,297
Recurring \| Level 2 \| Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	348,526	384,633
Recurring \| Level 2 \| Corporate debt securities
Available-for-sale investment securities
Fair Value	7,394	7,114
Recurring \| Level 2 \| Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	8,926	12,270
Recurring \| Level 3
Available-for-sale investment securities
Fair Value	0	0
Equity securities	0	0
Mortgage loans held for sale	0
Total	2	5
Recurring \| Level 3 \| Interest rate swap agreements
Available-for-sale investment securities
Derivative asset		0
Derivative liability		0
Recurring \| Level 3 \| Forward sales commitments
Available-for-sale investment securities
Derivative asset	0	0
Recurring \| Level 3 \| Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	2	5
Recurring \| Level 3 \| U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 3 \| Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 3 \| Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 3 \| Corporate debt securities
Available-for-sale investment securities
Fair Value	0	0
Recurring \| Level 3 \| Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	$ 0	$ 0

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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