INVESTMENT SECURITIES, Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 2,957,819	$ 3,007,148
Gross unrealized gains	6,572	33,501
Gross unrealized losses	59,612	9,561
Fair value	2,904,779	3,031,088
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,957,819
Fair value	2,904,779
Marketable Equity Securities [Abstract]
Amortized cost	0	251
Gross unrealized gains	0	275
Gross unrealized losses	0	0
Fair value	0	526
U.S. Treasury and Agency Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,039,086	2,043,023
Gross unrealized gains	0	15,886
Gross unrealized losses	40,991	4,838
Fair value	1,998,095	2,054,071
Obligations of State and Political Subdivisions [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	473,048	514,949
Gross unrealized gains	5,250	14,064
Gross unrealized losses	2,933	57
Fair value	475,365	528,956
Government Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	369,970	358,180
Gross unrealized gains	1,297	3,121
Gross unrealized losses	13,204	3,763
Fair value	358,063	357,538
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,603	2,648
Gross unrealized gains	0	0
Gross unrealized losses	53	25
Fair value	2,550	2,623
Government Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	73,112	88,097
Gross unrealized gains	25	155
Gross unrealized losses	2,431	878
Fair value	$ 70,706	$ 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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-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Amount, before tax, of unrealized loss 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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