FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	$ 2,922,943
Equity securities	464	$ 432
Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	20
U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,034,972	2,023,753
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	433,159	459,154
Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	386,543	382,477
Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,553	2,546
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	65,716	68,119
Recurring [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,922,943	2,936,049
Equity securities	464	432
Mortgage loans held for sale	212	83
Total	2,923,763	2,936,615
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	20
Recurring [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative liability	(6)
Recurring [Member] | Interest Rate Swap Agreements [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	835	793
Derivative liability	(705)	(742)
Recurring [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,034,972	2,023,753
Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	433,159	459,154
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	386,543	382,477
Recurring [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,553	2,546
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	65,716	68,119
Recurring [Member] | Level 1 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,913,361	1,896,931
Equity securities	464	432
Mortgage loans held for sale	0	0
Total	1,913,825	1,897,363
Recurring [Member] | Level 1 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0
Recurring [Member] | Level 1 [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative liability	0
Recurring [Member] | Level 1 [Member] | Interest Rate Swap Agreements [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Derivative liability	0	0
Recurring [Member] | Level 1 [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,913,361	1,896,931
Recurring [Member] | 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] | Level 1 [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Level 1 [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Level 2 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,009,582	1,039,118
Equity securities	0	0
Mortgage loans held for sale	212	83
Total	1,009,918	1,039,252
Mortgage loans held for sale, at principal value	200	100
Recurring [Member] | Level 2 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0
Recurring [Member] | Level 2 [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative liability	(6)
Recurring [Member] | Level 2 [Member] | Interest Rate Swap Agreements [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	835	793
Derivative liability	(705)	(742)
Recurring [Member] | Level 2 [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	121,611	126,822
Recurring [Member] | Level 2 [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	433,159	459,154
Recurring [Member] | Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	386,543	382,477
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,553	2,546
Recurring [Member] | Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	65,716	68,119
Recurring [Member] | 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	20	0
Recurring [Member] | Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	20
Recurring [Member] | Level 3 [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative liability	0
Recurring [Member] | Level 3 [Member] | Interest Rate Swap Agreements [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Derivative liability	0	0
Recurring [Member] | 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] | 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] | Level 3 [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Level 3 [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	$ 0	$ 0