INVESTMENT SECURITIES, Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 2,972,889	$ 3,007,148
Gross unrealized gains	10,078	33,501
Gross unrealized losses	43,023	9,561
Fair value	2,939,944	3,031,088
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,972,889
Fair value	2,939,944
Marketable Equity Securities [Abstract]
Amortized cost		251
Gross unrealized gains		275
Gross unrealized losses		0
Fair value		526
U.S. Treasury and Agency Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,042,116	2,043,023
Gross unrealized gains	477	15,886
Gross unrealized losses	28,631	4,838
Fair value	2,013,962	2,054,071
Obligations of State and Political Subdivisions [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	485,099	514,949
Gross unrealized gains	7,825	14,064
Gross unrealized losses	1,440	57
Fair value	491,484	528,956
Government Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	364,919	358,180
Gross unrealized gains	1,724	3,121
Gross unrealized losses	10,746	3,763
Fair value	355,897	357,538
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,618	2,648
Gross unrealized gains	0	0
Gross unrealized losses	55	25
Fair value	2,563	2,623
Government Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	78,137	88,097
Gross unrealized gains	52	155
Gross unrealized losses	2,151	878
Fair value	$ 76,038	$ 87,374

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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

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