FAIR VALUE - Financial assets and liabilities accounted for at fair value on a recurring basis (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Available-for-sale investment securities
Available-for-sale investment securities	$ 4,151,851	$ 4,934,210
Equity securities	419	463
U.S. Treasury and agency securities
Available-for-sale investment securities
Available-for-sale investment securities	3,243,537	3,998,564
Obligations of state and political subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	504,297	430,289
Government agency mortgage-backed securities
Available-for-sale investment securities
Available-for-sale investment securities	384,633	477,056
Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	7,114	7,962
Government agency collateralized mortgage obligations
Available-for-sale investment securities
Available-for-sale investment securities	12,270	20,339
Level 3 | Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	5	51
Recurring
Available-for-sale investment securities
Available-for-sale investment securities	4,151,851	4,934,210
Equity securities	419	463
Total	4,152,280	4,935,049
Recurring | Interest rate swap agreements
Available-for-sale investment securities
Derivative asset	1	296
Derivative liability	(1)	(3)
Recurring | Forward sales commitments
Available-for-sale investment securities
Derivative asset		32
Derivative liability	5
Recurring | Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	5	51
Recurring | U.S. Treasury and agency securities
Available-for-sale investment securities
Available-for-sale investment securities	3,243,537	3,998,564
Recurring | Obligations of state and political subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	504,297	430,289
Recurring | Government agency mortgage-backed securities
Available-for-sale investment securities
Available-for-sale investment securities	384,633	477,056
Recurring | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	7,114	7,962
Recurring | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Available-for-sale investment securities	12,270	20,339
Recurring | Level 1
Available-for-sale investment securities
Available-for-sale investment securities	3,178,189	3,900,924
Equity securities	419	463
Total	3,178,608	3,901,387
Recurring | Level 1 | Interest rate swap agreements
Available-for-sale investment securities
Derivative asset	0	0
Derivative liability	0	0
Recurring | Level 1 | Forward sales commitments
Available-for-sale investment securities
Derivative asset		0
Derivative liability	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
Available-for-sale investment securities	3,178,189	3,900,924
Recurring | Level 1 | Obligations of state and political subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Level 1 | Government agency mortgage-backed securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Level 1 | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Level 1 | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Level 2
Available-for-sale investment securities
Available-for-sale investment securities	973,662	1,033,286
Equity securities	0	0
Total	973,667	1,033,611
Recurring | Level 2 | Interest rate swap agreements
Available-for-sale investment securities
Derivative asset	1	296
Derivative liability	(1)	(3)
Recurring | Level 2 | Forward sales commitments
Available-for-sale investment securities
Derivative asset		32
Derivative liability	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
Available-for-sale investment securities	65,348	97,640
Recurring | Level 2 | Obligations of state and political subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	504,297	430,289
Recurring | Level 2 | Government agency mortgage-backed securities
Available-for-sale investment securities
Available-for-sale investment securities	384,633	477,056
Recurring | Level 2 | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	7,114	7,962
Recurring | Level 2 | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Available-for-sale investment securities	12,270	20,339
Recurring | Level 3
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Equity securities	0	0
Total	5	51
Recurring | Level 3 | Interest rate swap agreements
Available-for-sale investment securities
Derivative asset	0	0
Derivative liability	0	0
Recurring | Level 3 | Forward sales commitments
Available-for-sale investment securities
Derivative asset		0
Derivative liability	0
Recurring | Level 3 | Commitments to originate real estate loans for sale
Available-for-sale investment securities
Derivative asset	5	51
Recurring | Level 3 | U.S. Treasury and agency securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Level 3 | Obligations of state and political subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Level 3 | Government agency mortgage-backed securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Level 3 | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Level 3 | Government agency collateralized mortgage obligations
Available-for-sale investment securities
Available-for-sale investment securities	$ 0	$ 0