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Fair Value Measurements (Narrative) (Details) - USD ($) shares in Millions, $ in Millions | 4 Months Ended | 12 Months Ended
--- | --- | ---
May 31, 2020 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Equity method investments |  | $ 207 | $ 280 |
Equity method income |  |  |  | $ 12
Gain (loss) on equity securities |  | 5 | 27 | $ 10
Other investments |  | 36 | $ 33 |
Alexander Forbes |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Sale of stock, number of shares issued in transaction (in shares) | 240 |  | 242 |
Private Insurance and Consulting |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Equity method investments |  | 58 | $ 169 |
Private Equity Investments |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Equity method investments |  | 149 | 111 |
Equity method income |  | 56 | 3 |
Equity Investments |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Investments, fair value |  | 75 | $ 72 |
Fair Value, Measurements, Recurring | Unobservable Inputs (Level 3) |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Proceeds from contingent purchase consideration, asset |  | $ 90 |  |
Minimum |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Revenue target period |  | 2 years |  |
Maximum |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |
Revenue target period |  | 4 years |  |

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