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Derivatives (Details) £ in Billions |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Sep. 20, 2018 GBP (£) | Jan. 31, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Mar. 31, 2019 EUR (€)
Derivative [Line Items] |  |  |  |  |  |  |
Increase in net investment hedges |  |  |  | $ 124,000,000 |  |  |
Purchase consideration (GBP) |  |  |  | $ 877,000,000 | $ 5,927,000,000 |  |
FX Contract |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Gain on derivative |  |  |  |  | 31,000,000 |  |
Unrealized loss |  |  |  |  |  | $ 325,000,000 |
Treasury Lock |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Charge to settle derivative contract |  | $ 122,000,000 | $ 6,000,000 |  |  |  |
JLT Transaction |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Purchase consideration (GBP) | £ | £ 5.2 |  |  |  |  |  |
JLT Transaction | FX Contract |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Loss due to change in fair value of derivative |  |  |  |  | 26,000,000 |  |
JLT Transaction | Treasury Lock |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Unrealized loss |  |  |  |  |  | 116,000,000 |
Derivative hedge, asset |  |  |  |  |  | $ 2,000,000,000 |
Senior Notes |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Debt instrument, face amount |  | $ 5,000,000,000 |  |  |  |  |
Senior Notes | JLT Transaction |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  | 6,500,000,000 |  | € 1,100,000,000
Net investment hedge, threshold percentage of the equity balance |  |  |  | 80.00% |  |  |
Charge to settle derivative contract |  |  |  |  | $ 7,300,000 |  |

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