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Investment Securities - Schedule of FHN's Investment Securities (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 9,133 |
Fair Value | 8,248 | $ 8,391
Amortized Cost | 1,282 | 1,323
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (138) | (162)
Fair Value | 1,144 | 1,161
Asset Pledged as Collateral |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,300 | 1,300
Government agency issued MBS |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 4,754 | 5,061
Gross Unrealized Gains | 5 | 2
Gross Unrealized Losses | (460) | (579)
Fair Value | 4,299 | 4,484
Amortized Cost | 814 | 852
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (77) | (96)
Fair Value | 737 | 756
Government agency issued CMO |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 2,530 | 2,487
Gross Unrealized Gains | 1 | 0
Gross Unrealized Losses | (276) | (341)
Fair Value | 2,255 | 2,146
Amortized Cost | 468 | 471
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (61) | (66)
Fair Value | 407 | 405
Other U.S. government agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,251 | 1,321
Gross Unrealized Gains | 5 | 2
Gross Unrealized Losses | (122) | (151)
Fair Value | 1,134 | 1,172
States and municipalities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 598 | 627
Gross Unrealized Gains | 1 | 3
Gross Unrealized Losses | (39) | (41)
Fair Value | 560 | 589
Securities available-for-sale, excluding interest only strip |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 9,133 | 9,496
Gross Unrealized Gains | 12 | 7
Gross Unrealized Losses | (897) | (1,112)
Fair Value | 8,248 | 8,391
Securities available-for-sale, excluding interest only strip | Asset Pledged as Collateral |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 7,200 | $ 7,600

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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://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 + Details
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