INVESTMENT SECURITIES, Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 3,129,978	$ 3,011,551
Gross unrealized gains	163,430	42,017
Gross unrealized losses	292	9,140
Fair value	3,293,116	3,044,428
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	3,129,978
Fair value	3,293,116
U.S. Treasury and Agency Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,048,199	2,030,060
Gross unrealized gains	124,122	21,674
Gross unrealized losses	18	7,975
Fair value	2,172,303	2,043,759
Obligations of State and Political Subdivisions [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	499,961	497,852
Gross unrealized gains	21,764	14,382
Gross unrealized losses	20	26
Fair value	521,705	512,208
Government Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	519,370	428,491
Gross unrealized gains	15,956	5,478
Gross unrealized losses	248	1,107
Fair value	535,078	432,862
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	4,509	2,527
Gross unrealized gains	68	1
Gross unrealized losses	0	0
Fair value	4,577	2,528
Government Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	57,939	52,621
Gross unrealized gains	1,520	482
Gross unrealized losses	6	32
Fair value	$ 59,453	$ 53,071

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
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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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 debt and equity securities classified as available-for-sale.

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Amount 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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
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