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Goodwill and Other Intangible Assets - Narrative (Details) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Jul. 01, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) | Jul. 17, 2020 USD ($) branch | Dec. 31, 2019 USD ($)
Goodwill [Line Items] |  |  |  |  |  |  |  |
Purchase accounting gain |  | $ 0 | $ 532 | $ (1) | $ 532 |  |  |
Goodwill |  | $ 1,511 |  | $ 1,511 |  | $ 1,511 |  | $ 1,433
IBERIABANK (IBKC) |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Purchase accounting gain | $ 531 |  |  |  |  | 531 |  |
Truist Bank Branches |  |  |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |  |  |
Number of bank branches | branch |  |  |  |  |  |  | 30 |
Goodwill |  |  |  |  |  | $ 78 | $ 78 |

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