FAIR VALUE - Financial assets and liabilities accounted for at fair value on a recurring basis (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2023	Dec. 31, 2022
Available-for-sale investment securities
Fair Value	$ 3,064,870	$ 4,151,851
Equity securities	369	419
Mortgage loans held for sale		0
Unpaid principal value of mortgage loans held for sale	1,400
Transfers between Level 2 and Level 1	0
Transfers between Level 1 and Level 2	0
Transfers in of level 3	0
Transfers Out of level 3	0
U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	2,189,845	3,243,537
Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	491,963	504,297
Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	365,506	384,633
Corporate debt securities
Available-for-sale investment securities
Fair Value	7,097	7,114
Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	10,459	12,270
Recurring
Available-for-sale investment securities
Fair Value	3,064,870	4,151,851
Equity securities	369	419
Mortgage loans held for sale	1,401
Total	3,066,679	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	21	5
Recurring | Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	18	5
Recurring | U.S. Treasury and agency securities
Available-for-sale investment securities
Fair Value	2,189,845	3,243,537
Recurring | Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	491,963	504,297
Recurring | Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	365,506	384,633
Recurring | Corporate debt securities
Available-for-sale investment securities
Fair Value	7,097	7,114
Recurring | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	10,459	12,270
Recurring | Level 1
Available-for-sale investment securities
Fair Value	2,125,720	3,178,189
Equity securities	369	419
Mortgage loans held for sale	0
Total	2,126,089	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	2,125,720	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	939,150	973,662
Equity securities	0	0
Mortgage loans held for sale	1,401
Total	940,572	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	21	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	64,125	65,348
Recurring | Level 2 | Obligations of state and political subdivisions
Available-for-sale investment securities
Fair Value	491,963	504,297
Recurring | Level 2 | Government agency mortgage-backed securities
Available-for-sale investment securities
Fair Value	365,506	384,633
Recurring | Level 2 | Corporate debt securities
Available-for-sale investment securities
Fair Value	7,097	7,114
Recurring | Level 2 | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Fair Value	10,459	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	18	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	18	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