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INVESTMENT SECURITIES - Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands | Dec. 31, 2020 USD ($) position | Dec. 31, 2019 USD ($) position
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Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | position | 117 |
12 months or longer | position | 3 |
Total | position | 120 |
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 913,612 |
12 months or longer | 14 |
Total | 913,626 |
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 16,788 |
12 months or longer | 0 |
Total | 16,788 |
Available-for-sale, allowance for credit losses | $ 0 |
U.S. Treasury and Agency Securities [Member] |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | position | 13 | 12
12 months or longer | position | 0 | 5
Total | position | 13 | 17
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 831,015 | $ 592,678
12 months or longer | 0 | 25,998
Total | 831,015 | 618,676
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 16,595 | 7,970
12 months or longer | 0 | 5
Total | $ 16,595 | $ 7,975
Obligations of State and Political Subdivisions [Member] |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | position | 1 | 21
12 months or longer | position | 0 | 0
Total | position | 1 | 21
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 358 | $ 22,716
12 months or longer | 0 | 0
Total | 358 | 22,716
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 0 | 26
12 months or longer | 0 | 0
Total | $ 0 | $ 26
Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | position | 89 | 50
12 months or longer | position | 2 | 52
Total | position | 91 | 102
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 75,992 | $ 89,237
12 months or longer | 14 | 52,975
Total | 76,006 | 142,212
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 182 | 341
12 months or longer | 0 | 766
Total | $ 182 | $ 1,107
Corporate Debt Securities [Member] |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | position | 1 | 5
12 months or longer | position | 0 | 5
Total | position | 1 | 10
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 1,001 | $ 5,971
12 months or longer | 0 | 4,405
Total | 1,001 | 10,376
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 1 | 14
12 months or longer | 0 | 18
Total | $ 1 | $ 32
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | position | 13 | 88
12 months or longer | position | 1 | 62
Total | position | 14 | 150
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 5,246 | $ 710,602
12 months or longer | 0 | 83,378
Total | 5,246 | 793,980
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 10 | 8,351
12 months or longer | 0 | 789
Total | $ 10 | $ 9,140

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- Definition Amount of allowance for credit loss 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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aaa) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 Reference 4: 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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