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Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, numberOfColleague in Thousands, shares in Millions, $ in Millions, £ in Billions |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 01, 2019 USD ($) | Apr. 01, 2019 USD ($) numberOfColleague $ / £ | Apr. 01, 2019 GBP (£) numberOfColleague £ / shares $ / £ | Sep. 20, 2018 GBP (£) | May 31, 2020 shares | Feb. 29, 2020 shares | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) shares | Dec. 31, 2020 USD ($) acquisition shares | Dec. 31, 2019 USD ($) acquisition shares | Dec. 31, 2018 USD ($) | Jan. 01, 2019
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total purchase consideration |  |  |  |  |  |  |  |  | $ 877 | $ 5,927 |  |
Cash paid |  |  |  |  |  |  |  |  | 694 | 5,861 |  |
Estimated contingent consideration |  |  |  |  |  |  |  |  | 183 | 66 | $ 133 |
Deferred purchase consideration |  |  |  |  |  |  |  |  | 122 | 65 | 117 |
Revenue related to acquisitions |  |  |  |  |  |  |  |  | 169 | 1,200 | 120 |
Net operating income (loss) related to acquisitions |  |  |  |  |  |  |  |  | 11 | (40) | 2 |
Acquisition-related expenses incurred |  |  |  |  |  |  |  |  | 3 |  | 7 |
Long-term indebtedness assumed |  |  |  |  |  |  |  |  | 78 | 2,804 | 83 |
Cash payments for subsidiaries |  |  |  |  |  |  |  |  | $ 0 | 91 | $ 0 |
Increase (decrease) in costs |  |  |  |  |  |  |  |  |  | $ 207 |  |
Risk and insurance services |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of acquisitions made | acquisition |  |  |  |  |  |  |  |  | 7 | 5 |  |
Certain Businesses Primarily in the U.S., U.K. and Canada |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash proceeds from sale |  |  |  |  |  |  |  |  | $ 98 |  |  |
Alexander Forbes |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Investment owned, balance (in shares) | shares |  |  |  |  |  |  |  |  | 201 |  |  |
U.S. Specialty business and U.S. large market health and defined benefit business | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash proceeds from sale |  |  |  |  |  |  | $ 60 |  |  |  |  |
JLT’s global aerospace business | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash proceeds from sale | $ 165 |  |  |  |  |  |  |  |  |  |  |
Contingent consideration receivable | $ 65 |  |  |  |  |  |  |  |  |  |  |
Alexander Forbes |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Equity investment shares owned by the Company (in shares) | shares |  |  |  |  |  |  |  | 443 |  | 443 |  |
Sale of stock, number of shares issued in transaction (in shares) | shares |  |  |  |  | 193 | 49 |  |  |  |  |  |
Marsh India |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of interest acquired |  |  |  |  |  |  |  |  |  |  | 26.00% | 49.00%
Acquisitions made in prior years |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated contingent consideration |  |  |  |  |  |  |  |  | $ 102 | $ 63 |  |
Deferred purchase consideration |  |  |  |  |  |  |  |  | $ 68 | 43 |  |
JLT Transaction |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total purchase consideration | £ |  |  |  | £ 5.2 |  |  |  |  |  |  |  |
Acquisition-related expenses incurred |  |  |  |  |  |  |  |  |  | $ 125 |  |
Share price (in GBP per share) | £ / shares |  |  | £ 19.15 |  |  |  |  |  |  |  |  |
Consideration transferred, equity interests issued and issuable |  | $ 5,600 | £ 4.3 |  |  |  |  |  |  |  |  |
Exchange rate (in USD per GBP) | $ / £ |  | 1.31 | 1.31 |  |  |  |  |  |  |  |  |
Long-term indebtedness assumed |  | $ 1,000 |  |  |  |  |  |  |  |  |  |
Number of colleagues | numberOfColleague |  | 10 | 10 |  |  |  |  |  |  |  |  |
Cash payments for subsidiaries |  |  |  |  |  |  |  | $ 79 |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue target period |  |  |  |  |  |  |  |  | 2 years |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue target period |  |  |  |  |  |  |  |  | 4 years |  |  |

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