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FAIR VALUE - Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	$ 4,358,371
Equity securities	459	$ 445
Derivative asset		14
U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	3,393,260	2,501,382
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	421,965	475,660
Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	510,676	522,638
Corporate debt securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	8,078	4,635
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	24,392	43,577
Significant Unobservable Inputs, Level 3 [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	45
Recurring [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	4,358,371	3,547,892
Equity securities	459	445
Mortgage loans held for sale	117	1,622
Total	4,359,375	3,550,473
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities [Abstract]
Derivative asset	45	14
Recurring [Member] | Forward Sales Commitments
Available-for-sale investment securities [Abstract]
Derivative asset	53	2
Recurring [Member] | Interest Rate Swap Agreements
Available-for-sale investment securities [Abstract]
Derivative asset	335	1,572
Derivative liability	(5)	1,074
Recurring [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	3,393,260	2,501,382
Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	421,965	475,660
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	510,676	522,638
Recurring [Member] | Corporate debt securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	8,078	4,635
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	24,392	43,577
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	3,293,134	2,359,912
Equity securities	459	445
Mortgage loans held for sale	0	0
Total	3,293,593	2,360,357
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Forward Sales Commitments
Available-for-sale investment securities [Abstract]
Derivative asset	0	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	3,293,134	2,359,912
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
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	1,065,237	1,187,980
Equity securities	0	0
Mortgage loans held for sale	117	1,622
Total	1,065,737	1,190,102
Mortgage loans held for sale	100	1,600
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Forward Sales Commitments
Available-for-sale investment securities [Abstract]
Derivative asset	53	2
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Interest Rate Swap Agreements
Available-for-sale investment securities [Abstract]
Derivative asset	335	1,572
Derivative liability	(5)	1,074
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	100,126	141,470
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	421,965	475,660
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	510,676	522,638
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Corporate debt securities
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	8,078	4,635
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	24,392	43,577
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	0
Total	45	14
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale
Available-for-sale investment securities [Abstract]
Derivative asset	45	14
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Forward Sales Commitments
Available-for-sale investment securities [Abstract]
Derivative asset	0	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
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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