INVESTMENT SECURITIES, Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 2,986,577	$ 3,011,551
Gross unrealized gains	155,321	42,017
Gross unrealized losses	16	9,140
Fair value	3,141,882	3,044,428
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,986,577
Fair value	3,141,882
U.S. Treasury and Agency Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,016,500	2,030,060
Gross unrealized gains	122,579	21,674
Gross unrealized losses	0	7,975
Fair value	2,139,079	2,043,759
Obligations of State and Political Subdivisions [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	485,755	497,852
Gross unrealized gains	15,955	14,382
Gross unrealized losses	2	26
Fair value	501,708	512,208
Government Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	433,129	428,491
Gross unrealized gains	15,129	5,478
Gross unrealized losses	10	1,107
Fair value	448,248	432,862
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,512	2,527
Gross unrealized gains	0	1
Gross unrealized losses	4	0
Fair value	2,508	2,528
Government Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	48,681	52,621
Gross unrealized gains	1,658	482
Gross unrealized losses	0	32
Fair value	$ 50,339	$ 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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