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Investment Securities - Schedule of FHN's Investment Securities (Details) - USD ($) $ in Millions | Mar. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 8,956 |
Fair Value | 8,075 | $ 7,896
Amortized Cost | 1,258 | 1,270
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (166) | (187)
Fair Value | 1,092 | 1,083
Asset Pledged as Collateral |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,200 | 1,300
Government agency issued MBS |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 4,183 | 4,223
Gross Unrealized Gains | 3 | 1
Gross Unrealized Losses | (436) | (522)
Fair Value | 3,750 | 3,702
Amortized Cost | 793 | 804
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (95) | (109)
Fair Value | 698 | 695
Government agency issued CMO |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 3,192 | 3,079
Gross Unrealized Gains | 1 | 0
Gross Unrealized Losses | (280) | (312)
Fair Value | 2,913 | 2,767
Amortized Cost | 465 | 466
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (71) | (78)
Fair Value | 394 | 388
Other U.S. government agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,200 | 1,234
Gross Unrealized Gains | 2 | 0
Gross Unrealized Losses | (129) | (161)
Fair Value | 1,073 | 1,073
States and municipalities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 381 | 394
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (42) | (40)
Fair Value | 339 | 354
Securities available-for-sale, excluding interest only strip |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 8,956 | 8,930
Gross Unrealized Gains | 6 | 1
Gross Unrealized Losses | (887) | (1,035)
Fair Value | 8,075 | 7,896
Securities available-for-sale, excluding interest only strip | Asset Pledged as Collateral |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 7,000 | $ 6,900

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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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