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FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale investment securities [Abstract]
Fair value, assets, levels transfers	$ 0
Available-for-sale investment securities	3,547,892
Equity securities	445	$ 451
U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,501,382	2,043,759
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	475,660	512,208
Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	522,638	432,862
Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	4,635	2,528
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	43,577	53,071
Significant Unobservable Inputs, Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	14
Recurring [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	3,547,892	3,044,428
Equity securities	445	451
Mortgage loans held for sale	1,622
Total	3,550,473	3,045,144
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	14
Recurring [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative liability	2
Recurring [Member] | Interest Rate Swap Agreements
Available-for-sale investment securities [Abstract]
Derivative asset	1,572	851
Derivative liability	(1,074)	(586)
Recurring [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,501,382	2,043,759
Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	475,660	512,208
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	522,638	432,862
Recurring [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	4,635	2,528
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	43,577	53,071
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,359,912	1,878,705
Equity securities	445	451
Mortgage loans held for sale	0
Total	2,360,357	1,879,156
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative liability	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Interest Rate Swap Agreements
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Derivative liability	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,359,912	1,878,705
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Observable Inputs, Level 2 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,187,980	1,165,723
Equity securities	0	0
Mortgage loans held for sale	1,622
Total	1,190,102	1,165,988
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative liability	2
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Interest Rate Swap Agreements
Available-for-sale investment securities [Abstract]
Derivative asset	1,572	851
Derivative liability	(1,074)	(586)
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	141,470	165,054
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	475,660	512,208
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	522,638	432,862
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	4,635	2,528
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	43,577	53,071
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Equity securities	0	0
Mortgage loans held for sale	0
Total	14	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	14
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative liability	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Interest Rate Swap Agreements
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Derivative liability	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	$ 0	$ 0

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