FAIR VALUE - Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($)	Dec. 31, 2021	Dec. 31, 2020
Available-for-sale investment securities
Available-for-sale investment securities	$ 4,934,210,000
Equity securities	463,000	$ 445,000
U.S. Treasury and Agency Securities
Available-for-sale investment securities
Available-for-sale investment securities	3,998,564,000	2,501,382,000
Obligations of State and Political Subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	430,289,000	475,660,000
Government Agency Mortgage-Backed Securities
Available-for-sale investment securities
Available-for-sale investment securities	477,056,000	522,638,000
Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	7,962,000	4,635,000
Government Agency Collateralized Mortgage Obligations
Available-for-sale investment securities
Available-for-sale investment securities	20,339,000	43,577,000
Significant Unobservable Inputs, Level 3 | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities
Derivative asset	51,000	14,000
Recurring
Available-for-sale investment securities
Available-for-sale investment securities	4,934,210,000	3,547,892,000
Equity securities	463,000	445,000
Mortgage loans held for sale		1,622,000
Total	4,935,049,000	3,550,473,000
Recurring | Interest Rate Swap Agreements
Available-for-sale investment securities
Derivative asset	296,000	1,572,000
Derivative liability	(3,000)	(1,074,000)
Recurring | ForwardContractsMember
Available-for-sale investment securities
Derivative asset		2,000
Derivative liability	32,000
Recurring | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities
Derivative asset	51,000	14,000
Recurring | U.S. Treasury and Agency Securities
Available-for-sale investment securities
Available-for-sale investment securities	3,998,564,000	2,501,382,000
Recurring | Obligations of State and Political Subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	430,289,000	475,660,000
Recurring | Government Agency Mortgage-Backed Securities
Available-for-sale investment securities
Available-for-sale investment securities	477,056,000	522,638,000
Recurring | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	7,962,000	4,635,000
Recurring | Government Agency Collateralized Mortgage Obligations
Available-for-sale investment securities
Available-for-sale investment securities	20,339,000	43,577,000
Recurring | Quoted Prices in Active Markets For Identical Assets, Level 1
Available-for-sale investment securities
Available-for-sale investment securities	3,900,924,000	2,359,912,000
Equity securities	463,000	445,000
Mortgage loans held for sale		0
Total	3,901,387,000	2,360,357,000
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
Derivative liability	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	3,900,924,000	2,359,912,000
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	1,033,286,000	1,187,980,000
Equity securities	0	0
Mortgage loans held for sale	0	1,622,000
Total	1,033,611,000	1,190,102,000
Recurring | Significant Observable Inputs, Level 2 | Interest Rate Swap Agreements
Available-for-sale investment securities
Derivative asset	296,000	1,572,000
Derivative liability	(3,000)	(1,074,000)
Recurring | Significant Observable Inputs, Level 2 | ForwardContractsMember
Available-for-sale investment securities
Derivative asset		2,000
Derivative liability	32,000
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	97,640,000	141,470,000
Recurring | Significant Observable Inputs, Level 2 | Obligations of State and Political Subdivisions
Available-for-sale investment securities
Available-for-sale investment securities	430,289,000	475,660,000
Recurring | Significant Observable Inputs, Level 2 | Government Agency Mortgage-Backed Securities
Available-for-sale investment securities
Available-for-sale investment securities	477,056,000	522,638,000
Recurring | Significant Observable Inputs, Level 2 | Corporate debt securities
Available-for-sale investment securities
Available-for-sale investment securities	7,962,000	4,635,000
Recurring | Significant Observable Inputs, Level 2 | Government Agency Collateralized Mortgage Obligations
Available-for-sale investment securities
Available-for-sale investment securities	20,339,000	43,577,000
Recurring | Significant Unobservable Inputs, Level 3
Available-for-sale investment securities
Available-for-sale investment securities	0	0
Equity securities	0	0
Mortgage loans held for sale		0
Total	51,000	14,000
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
Derivative liability	0
Recurring | Significant Unobservable Inputs, Level 3 | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities
Derivative asset	51,000	14,000
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