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Investment Securities - Schedule of FHN's Investment Securities (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 8,838 | $ 8,930
Gross Unrealized Gains | 14 | 1
Gross Unrealized Losses | (750) | (1,035)
Fair Value | 8,102 | 7,896
Amortized Cost | 1,229 | 1,270
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (150) | (187)
Fair Value | 1,079 | 1,083
Asset Pledged as Collateral |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 7,000 | 6,900
Amortized Cost | 900 | 1,300
Government agency issued MBS |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 4,039 | 4,223
Gross Unrealized Gains | 7 | 1
Gross Unrealized Losses | (364) | (522)
Fair Value | 3,682 | 3,702
Amortized Cost | 769 | 804
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (81) | (109)
Fair Value | 688 | 695
Government agency issued CMO |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 3,078 | 3,079
Gross Unrealized Gains | 4 | 0
Gross Unrealized Losses | (239) | (312)
Fair Value | 2,843 | 2,767
Amortized Cost | 460 | 466
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (69) | (78)
Fair Value | 391 | 388
Other U.S. government agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,355 | 1,234
Gross Unrealized Gains | 2 | 0
Gross Unrealized Losses | (117) | (161)
Fair Value | 1,240 | 1,073
States and municipalities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 366 | 394
Gross Unrealized Gains | 1 | 0
Gross Unrealized Losses | (30) | (40)
Fair Value | $ 337 | $ 354

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