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Fair Value Measurements (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
May 31, 2020 | Feb. 29, 2020 | Apr. 30, 2019 | Mar. 31, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Increase in fair value of contingent consideration due to 5% increase in projections |  |  |  |  |  |  | $ 12 |  |
Net increase in estimated fair value of liabilities related to prior period acquisitions |  |  |  |  |  |  | 11 | $ 70 |
Decrease in fair value of contingent consideration due to 5% decrease in projections |  |  |  |  |  |  | 21 |  |
Equity method investments |  |  |  |  |  |  | 280 | 434 |
Equity method income |  |  |  |  |  |  |  | 12 | $ 17
Gain (loss) on equity securities |  |  |  |  |  |  | (27) | 10 | $ 54
Other investments |  |  |  |  |  |  | $ 33 | $ 67 |
Alexander Forbes |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Investment owned, balance (in shares) |  |  |  |  |  |  | 201 |  |
Alexander Forbes |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Equity investment shares owned by the Company (in shares) |  |  |  |  |  |  |  | 443 |
Sale of stock, number of shares issued in transaction (in shares) | 193 | 49 |  |  |  |  |  |  |
Investments, fair value |  |  |  |  |  |  | $ 54 |  |
Benefitfocus |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from sale of other investments |  |  | $ 17 | $ 132 |  | $ 115 |  |  |
Payscale |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from sale of other investments |  |  |  |  | $ 47 |  |  |  |
Private Insurance and Consulting |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  |  | 169 | $ 183 |
Private Equity Investments |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  |  | 111 | 107 |
Equity method income |  |  |  |  |  |  | 3 | 13 |
Equity Investments |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  |  |  |  | 72 | 19 |
Other Investments |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Equity securities without readily determinable fair value, upward price adjustment, annual amount |  |  |  |  |  |  | $ 2 |  |
Equity securities without readily determinable fair value, downward price adjustment, annual amount |  |  |  |  |  |  |  | $ 1 |
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 |  |
Money market funds |  |  |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |  |  |
Share price (in dollars per share) |  |  |  |  |  |  | $ 1.00 |  |

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