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INVESTMENT SECURITIES - Available-for-Sale Portfolio (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Available-for-Sale Portfolio [Abstract]
Amortized cost	$ 4,406,878	$ 3,427,779
Gross unrealized gains	73,896	136,901
Gross unrealized losses	122,403	16,788
Fair value	4,358,371	3,547,892
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	4,406,878
Fair value	4,358,371
U.S. Treasury and Agency Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	3,467,053	2,423,236
Gross unrealized gains	44,279	94,741
Gross unrealized losses	118,072	16,595
Fair value	3,393,260	2,501,382
Obligations of State and Political Subdivisions [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	404,531	451,028
Gross unrealized gains	17,654	24,632
Gross unrealized losses	220	0
Fair value	421,965	475,660
Government Agency Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	503,548	506,540
Gross unrealized gains	11,232	16,280
Gross unrealized losses	4,104	182
Fair value	510,676	522,638
Corporate debt securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	8,000	4,499
Gross unrealized gains	84	137
Gross unrealized losses	6	1
Fair value	8,078	4,635
Government Agency Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	23,746	42,476
Gross unrealized gains	647	1,111
Gross unrealized losses	1	10
Fair value	$ 24,392	$ 43,577

X

- Definition

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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