FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	$ 2,987,290	$ 3,031,088
Recurring [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,987,290	3,031,088
Mortgage loans held for sale	628	461
Total	2,988,027	3,031,802
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	55	89
Recurring [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	10	4
Recurring [Member] | Interest Rate Swap Agreements [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	937	1,064
Derivative liability	(893)	(904)
Recurring [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,027,914	2,054,071
Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	510,308	528,956
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	364,539	357,538
Recurring [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,579	2,623
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	81,429	87,374
Recurring [Member] | Marketable Equity Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	521	526
Recurring [Member] | Level 1 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,885,536	1,909,816
Mortgage loans held for sale	0	0
Total	1,885,536	1,909,816
Recurring [Member] | Level 1 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Recurring [Member] | Level 1 [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0	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	1,885,015	1,909,290
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 1 [Member] | Marketable Equity Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	521	526
Recurring [Member] | Level 2 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,101,754	1,121,272
Mortgage loans held for sale	628	461
Total	1,102,436	1,121,897
Mortgage loans held for sale, at principal value	600
Recurring [Member] | Level 2 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0	0
Recurring [Member] | Level 2 [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	10	4
Recurring [Member] | Level 2 [Member] | Interest Rate Swap Agreements [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	937	1,064
Derivative liability	(893)	(904)
Recurring [Member] | Level 2 [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	142,899	144,781
Recurring [Member] | Level 2 [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	510,308	528,956
Recurring [Member] | Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	364,539	357,538
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,579	2,623
Recurring [Member] | Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	81,429	87,374
Recurring [Member] | Level 2 [Member] | Marketable Equity Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Recurring [Member] | Level 3 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0	0
Mortgage loans held for sale	0	0
Total	55	89
Recurring [Member] | Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	55	89
Recurring [Member] | Level 3 [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	0	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
Recurring [Member] | Level 3 [Member] | Marketable Equity Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	$ 0	$ 0