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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

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- 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). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 + Details
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