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Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Securities available for sale, amortized cost | $ 7,812,737 |
Securities available-for-sale | 7,995,872 | $ 4,445,403
Securities pledged as collateral | 5,700,000 | 3,800,000
U.S. treasuries |  |
Debt Securities, Available-for-sale [Line Items] |  |
Securities available for sale, amortized cost | 300,037 | 100
Securities available for sale, gross unrealized gains | 0 | 0
Securities available for sale, gross unrealized losses | (3) | 0
Securities available-for-sale | 300,034 | 100
Government agency issued MBS |  |
Debt Securities, Available-for-sale [Line Items] |  |
Securities available for sale, amortized cost | 3,731,490 | 2,316,381
Securities available for sale, gross unrealized gains | 97,734 | 34,692
Securities available for sale, gross unrealized losses | (2,158) | (2,556)
Securities available-for-sale | 3,827,066 | 2,348,517
Government agency issued CMO |  |
Debt Securities, Available-for-sale [Line Items] |  |
Securities available for sale, amortized cost | 2,675,192 | 1,667,773
Securities available for sale, gross unrealized gains | 31,694 | 9,916
Securities available for sale, gross unrealized losses | (1,439) | (7,197)
Securities available-for-sale | 2,705,447 | 1,670,492
Other U.S. government agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Securities available for sale, amortized cost | 644,328 | 303,463
Securities available for sale, gross unrealized gains | 13,942 | 3,750
Securities available for sale, gross unrealized losses | (4) | (1,121)
Securities available-for-sale | 658,266 | 306,092
Corporates and other debt |  |
Debt Securities, Available-for-sale [Line Items] |  |
Securities available for sale, amortized cost | 39,936 | 40,054
Securities available for sale, gross unrealized gains | 74 | 486
Securities available for sale, gross unrealized losses | 0 | 0
Securities available-for-sale | 40,010 | 40,540
States and municipalities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Securities available for sale, amortized cost | 421,754 | 57,232
Securities available for sale, gross unrealized gains | 10,342 | 3,324
Securities available for sale, gross unrealized losses | (6) | (30)
Securities available-for-sale | 432,090 | 60,526
Securities available-for-sale, excluding interest only strip: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Securities available for sale, amortized cost | 7,812,737 | 4,385,003
Securities available for sale, gross unrealized gains | 153,786 | 52,168
Securities available for sale, gross unrealized losses | (3,610) | (10,904)
Securities available-for-sale | 7,962,913 | 4,426,267
Interest- only strips- AFS |  |
Debt Securities, Available-for-sale [Line Items] |  |
Securities available-for-sale | $ 32,959 | $ 19,136

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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