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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

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). + 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=121645371&loc=d3e27161-111563 + Details
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- Definition 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). + 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 (c) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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- Definition Amortized cost of 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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 + Details
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