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INVESTMENT SECURITIES - Unrealized Gain/Loss on Investment (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
AFS |  |
Amortized cost | $ 950,878 | $ 1,027,338
Gross unrealized gains | 31,028 | 45,568
Gross unrealized losses | (3,063) | (813)
Fair value of debt securities available for sale | 978,843 | 1,072,093
Residential mortgage-backed securities |  |
AFS |  |
Amortized cost | 33,046 | 50,001
Gross unrealized gains | 719 | 1,237
Gross unrealized losses | (298) | (192)
Fair value of debt securities available for sale | 33,467 | 51,046
Commercial mortgage-backed securities |  |
AFS |  |
Amortized cost | 40,995 | 43,061
Gross unrealized gains | 1,231 | 2,131
Gross unrealized losses | (176) | (8)
Fair value of debt securities available for sale | 42,050 | 45,184
Residential collateralized mortgage obligations |  |
AFS |  |
Amortized cost | 182,060 | 228,685
Gross unrealized gains | 3,283 | 6,319
Gross unrealized losses | (983) | (95)
Fair value of debt securities available for sale | 184,360 | 234,909
Commercial collateralized mortgage obligations |  |
AFS |  |
Amortized cost | 139,202 | 155,645
Gross unrealized gains | 2,368 | 3,719
Gross unrealized losses | (246) | (181)
Fair value of debt securities available for sale | 141,324 | 159,183
Municipal bonds |  |
AFS |  |
Amortized cost | 513,151 | 533,719
Gross unrealized gains | 22,558 | 31,321
Gross unrealized losses | (1,308) | (337)
Fair value of debt securities available for sale | 534,401 | 564,703
HTM |  |
Amortized cost | 4,195 | 4,271
Gross unrealized gains | 154 | 236
Gross unrealized losses | 0 | 0
Fair value | 4,349 | 4,507
Corporate debt securities |  |
AFS |  |
Amortized cost | 18,990 | 14,381
Gross unrealized gains | 859 | 841
Gross unrealized losses | 0 | 0
Fair value of debt securities available for sale | 19,849 | 15,222
U.S. Treasury securities |  |
AFS |  |
Amortized cost | 23,434 |
Gross unrealized gains | 10 |
Gross unrealized losses | (52) |
Fair value of debt securities available for sale | $ 23,392 |
Agency debentures |  |
AFS |  |
Amortized cost |  | 1,846
Gross unrealized gains |  | 0
Gross unrealized losses |  | 0
Fair value of debt securities available for sale |  | $ 1,846

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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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