INVESTMENT SECURITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 3,005,165	$ 3,007,148
Gross unrealized gains	13,580	33,501
Gross unrealized losses	31,455	9,561
Fair value	2,987,290	3,031,088
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	3,004,915
Fair value	2,986,769
Other Securities [Abstract]
Amortized cost	45,352	50,291
Fair value	45,352	50,291
U.S. Treasury and Agency Securities [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	2,044,795	2,043,023
Gross unrealized gains	1,962	15,886
Gross unrealized losses	18,843	4,838
Fair value	2,027,914	2,054,071
Obligations of State and Political Subdivisions [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	502,517	514,949
Gross unrealized gains	9,126	14,064
Gross unrealized losses	1,335	57
Fair value	510,308	528,956
Government Agency Mortgage-Backed Securities [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	371,857	358,180
Gross unrealized gains	2,132	3,121
Gross unrealized losses	9,450	3,763
Fair value	364,539	357,538
Corporate Debt Securities [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	2,633	2,648
Gross unrealized gains	0	0
Gross unrealized losses	54	25
Fair value	2,579	2,623
Government Agency Collateralized Mortgage Obligations [Member]
Available For Sale Debt Securities Amortized Cost Basis [Abstract]
Amortized cost	83,113	88,097
Gross unrealized gains	89	155
Gross unrealized losses	1,773	878
Fair value	81,429	87,374
Marketable Equity Securities [Member]
Available For Sale Equity Securities Amortized Cost Basis [Abstract]
Amortized cost	250	251
Gross unrealized gains	271	275
Gross unrealized losses	0	0
Fair value	521	526
Federal Home Loan Bank Common Stock [Member]
Other Securities [Abstract]
Amortized cost	8,801	9,896
Fair value	8,801	9,896
Federal Reserve Bank Common Stock [Member]
Other Securities [Abstract]
Amortized cost	30,690	30,690
Fair value	30,690	30,690
Certificates of Deposit [Member]
Other Securities [Abstract]
Amortized cost	0	3,865
Fair value	0	3,865
Other Equity Securities [Member]
Other Securities [Abstract]
Amortized cost	5,861	5,840
Fair value	$ 5,861	$ 5,840

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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