INVESTMENT SECURITIES, Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 3,011,551	$ 2,952,278
Gross unrealized gains	42,017	10,339
Gross unrealized losses	9,140	26,568
Fair value	3,044,428	2,936,049
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	3,011,551
Fair value	3,044,428
U.S. Treasury and Agency Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,030,060	2,036,474
Gross unrealized gains	21,674	2,190
Gross unrealized losses	7,975	14,911
Fair value	2,043,759	2,023,753
Obligations of State and Political Subdivisions [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	497,852	453,640
Gross unrealized gains	14,382	6,563
Gross unrealized losses	26	1,049
Fair value	512,208	459,154
Government Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	428,491	390,234
Gross unrealized gains	5,478	1,526
Gross unrealized losses	1,107	9,283
Fair value	432,862	382,477
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,527	2,588
Gross unrealized gains	1	0
Gross unrealized losses	0	42
Fair value	2,528	2,546
Government Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	52,621	69,342
Gross unrealized gains	482	60
Gross unrealized losses	32	1,283
Fair value	$ 53,071	$ 68,119

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 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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