INVESTMENT SECURITIES - Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 3,427,779	$ 3,011,551
Gross unrealized gains	136,901	42,017
Gross unrealized losses	16,788	9,140
Fair value	3,547,892	3,044,428
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	3,427,779
Fair value	3,547,892
U.S. Treasury and Agency Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	2,423,236	2,030,060
Gross unrealized gains	94,741	21,674
Gross unrealized losses	16,595	7,975
Fair value	2,501,382	2,043,759
Obligations of State and Political Subdivisions [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	451,028	497,852
Gross unrealized gains	24,632	14,382
Gross unrealized losses	0	26
Fair value	475,660	512,208
Government Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	506,540	428,491
Gross unrealized gains	16,280	5,478
Gross unrealized losses	182	1,107
Fair value	522,638	432,862
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	4,499	2,527
Gross unrealized gains	137	1
Gross unrealized losses	1	0
Fair value	4,635	2,528
Government Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	42,476	52,621
Gross unrealized gains	1,111	482
Gross unrealized losses	10	32
Fair value	43,577	53,071
Federal Reserve Bank Stock [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Gross unrealized losses	0	0
Other Equity Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Gross unrealized losses	0	0
Federal Home Loan Bank Common Stock [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Gross unrealized losses	$ 0	$ 0

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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-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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