FAIR VALUE - Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Available-for-sale investment securities
Available-for-sale investment securities	$ 5,590,427	$ 4,934,210
Equity securities	443	463
Derivative asset		51
U.S. Treasury and Agency Securities
Available-for-sale investment securities
Available-for-sale investment securities	4,615,546	3,998,564
Obligations of State and Political Subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	527,155	430,289
Government Agency Mortgage-Backed Securities
Available-for-sale investment securities
Available-for-sale investment securities	425,357	477,056
Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	7,231	7,962
Government Agency Collateralized Mortgage Obligations
Available-for-sale investment securities
Available-for-sale investment securities	15,138	20,339
Significant Unobservable Inputs, Level 3
Available-for-sale investment securities
Derivative asset	6
Recurring
Available-for-sale investment securities
Available-for-sale investment securities	5,590,427	4,934,210
Equity securities	443	463
Total	5,590,905	4,935,049
Recurring | Interest Rate Swap Agreements
Available-for-sale investment securities
Derivative asset	8	296
Derivative liability	(2)	3
Recurring | ForwardContractsMember
Available-for-sale investment securities
Derivative asset	23	32
Recurring | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities
Derivative asset	6	51
Recurring | U.S. Treasury and Agency Securities
Available-for-sale investment securities
Available-for-sale investment securities	4,615,546	3,998,564
Recurring | Obligations of State and Political Subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	527,155	430,289
Recurring | Government Agency Mortgage-Backed Securities
Available-for-sale investment securities
Available-for-sale investment securities	425,357	477,056
Recurring | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	7,231	7,962
Recurring | Government Agency Collateralized Mortgage Obligations
Available-for-sale investment securities
Available-for-sale investment securities	15,138	20,339
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1
Available-for-sale investment securities
Available-for-sale investment securities	4,540,596	3,900,924
Equity securities	443	463
Total	4,541,039	3,901,387
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1 | Interest Rate Swap Agreements
Available-for-sale investment securities
Derivative asset	0	0
Derivative liability	0	0
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1 | ForwardContractsMember
Available-for-sale investment securities
Derivative asset	0	0
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1 | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities
Derivative asset	0	0
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1 | U.S. Treasury and Agency Securities
Available-for-sale investment securities
Available-for-sale investment securities	4,540,596	3,900,924
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1 | Obligations of State and Political Subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1 | Government Agency Mortgage-Backed Securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1 | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1 | Government Agency Collateralized Mortgage Obligations
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Significant Observable Inputs, Level 2
Available-for-sale investment securities
Available-for-sale investment securities	1,049,831	1,033,286
Equity securities	0	0
Total	1,049,860	1,033,611
Recurring | Significant Observable Inputs, Level 2 | Interest Rate Swap Agreements
Available-for-sale investment securities
Derivative asset	8	296
Derivative liability	(2)	3
Recurring | Significant Observable Inputs, Level 2 | ForwardContractsMember
Available-for-sale investment securities
Derivative asset	23	32
Recurring | Significant Observable Inputs, Level 2 | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities
Derivative asset	0	0
Recurring | Significant Observable Inputs, Level 2 | U.S. Treasury and Agency Securities
Available-for-sale investment securities
Available-for-sale investment securities	74,950	97,640
Recurring | Significant Observable Inputs, Level 2 | Obligations of State and Political Subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	527,155	430,289
Recurring | Significant Observable Inputs, Level 2 | Government Agency Mortgage-Backed Securities
Available-for-sale investment securities
Available-for-sale investment securities	425,357	477,056
Recurring | Significant Observable Inputs, Level 2 | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	7,231	7,962
Recurring | Significant Observable Inputs, Level 2 | Government Agency Collateralized Mortgage Obligations
Available-for-sale investment securities
Available-for-sale investment securities	15,138	20,339
Recurring | Significant Unobservable Inputs, Level 3
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Equity securities	0	0
Total	6	51
Recurring | Significant Unobservable Inputs, Level 3 | Interest Rate Swap Agreements
Available-for-sale investment securities
Derivative asset	0	0
Derivative liability	0	0
Recurring | Significant Unobservable Inputs, Level 3 | ForwardContractsMember
Available-for-sale investment securities
Derivative asset	0	0
Recurring | Significant Unobservable Inputs, Level 3 | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities
Derivative asset	6	51
Recurring | Significant Unobservable Inputs, Level 3 | U.S. Treasury and Agency Securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Significant Unobservable Inputs, Level 3 | Obligations of State and Political Subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Significant Unobservable Inputs, Level 3 | Government Agency Mortgage-Backed Securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Significant Unobservable Inputs, Level 3 | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Recurring | Significant Unobservable Inputs, Level 3 | Government Agency Collateralized Mortgage Obligations
Available-for-sale investment securities
Available-for-sale investment securities	$ 0	$ 0