INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 3,007,148	$ 2,706,863
Gross unrealized gains	33,501	47,463
Gross unrealized losses	9,561	5,670
Fair value	3,031,088	2,748,656
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	3,006,897
Estimated fair value	3,030,562
Other Securities [Abstract]
Amortized cost	50,291	35,736
Estimated fair value	50,291	35,736
U.S. Treasury and Agency Securities [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	2,043,023	1,876,358
Gross unrealized gains	15,886	28,522
Gross unrealized losses	4,838	2,118
Estimated fair value	2,054,071	1,902,762
Obligations of State and Political Subdivisions [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	514,949	582,655
Gross unrealized gains	14,064	13,389
Gross unrealized losses	57	1,054
Estimated fair value	528,956	594,990
Government Agency Mortgage-Backed Securities [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	358,180	232,657
Gross unrealized gains	3,121	5,040
Gross unrealized losses	3,763	2,467
Estimated fair value	357,538	235,230
Corporate Debt Securities [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	2,648	5,716
Gross unrealized gains	0	2
Gross unrealized losses	25	31
Estimated fair value	2,623	5,687
Government Agency Collateralized Mortgage Obligations [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	88,097	9,225
Gross unrealized gains	155	310
Gross unrealized losses	878	0
Estimated fair value	87,374	9,535
Marketable Equity Security [Member]
Available For Sale Equity Securities Amortized Cost Basis [Abstract]
Amortized cost	251	252
Gross unrealized gains	275	200
Gross unrealized losses	0	0
Estimated fair value	526	452
Federal Home Loan Bank Common Stock [Member]
Other Securities [Abstract]
Amortized cost	9,896	12,191
Estimated fair value	9,896	12,191
Federal Reserve Bank Common Stock [Member]
Other Securities [Abstract]
Amortized cost	30,690	19,781
Estimated fair value	30,690	19,781
Certificates of Deposit [Member]
Other Securities [Abstract]
Amortized cost	3,865	0
Estimated fair value	3,865	0
Other Equity Securities [Member]
Other Securities [Abstract]
Amortized cost	5,840	3,764
Estimated fair value	$ 5,840	$ 3,764

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