FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	$ 2,904,779
Equity securities	498	$ 0
Marketable equity securities	0	526
Available-for-sale investment securities	2,904,779	3,031,088
Derivative asset	9	89
U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,998,095	2,054,071
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	475,365	528,956
Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	358,063	357,538
Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,550	2,623
Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	70,706	87,374
Recurring [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,904,779
Equity securities	498
Marketable equity securities		526
Available-for-sale investment securities		3,031,088
Mortgage loans held for sale	0	461
Total	2,905,241	3,031,802
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	9	89
Recurring [Member] | Forward Sales Commitments [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	9	4
Recurring [Member] | Interest Rate Swap Agreements [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	1,306	1,064
Derivative liability	(1,360)	(904)
Recurring [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,998,095	2,054,071
Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	475,365	528,956
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	358,063	357,538
Recurring [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,550	2,623
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	70,706	87,374
Recurring [Member] | Level 1 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,867,144
Equity securities	498
Marketable equity securities		526
Available-for-sale investment securities		1,909,816
Mortgage loans held for sale	0	0
Total	1,867,642	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	1,867,144	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 2 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	1,037,635
Equity securities	0
Marketable equity securities		0
Available-for-sale investment securities		1,121,272
Mortgage loans held for sale	0	461
Total	1,037,590	1,121,897
Mortgage loans held for sale, at principal value	0
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	9	4
Recurring [Member] | Level 2 [Member] | Interest Rate Swap Agreements [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	1,306	1,064
Derivative liability	(1,360)	(904)
Recurring [Member] | Level 2 [Member] | U.S. Treasury and Agency Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	130,951	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	475,365	528,956
Recurring [Member] | Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	358,063	357,538
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	2,550	2,623
Recurring [Member] | Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	70,706	87,374
Recurring [Member] | Level 3 [Member]
Available-for-sale investment securities [Abstract]
Available-for-sale investment securities	0
Equity securities	0
Marketable equity securities		0
Available-for-sale investment securities		0
Mortgage loans held for sale	0	0
Total	9	89
Recurring [Member] | Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale [Member]
Available-for-sale investment securities [Abstract]
Derivative asset	9	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