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Acquisitions and Dispositions (Narrative) (Details) £ / shares in Units, $ in Millions, £ in Billions |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
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Jun. 01, 2019 USD ($) | Apr. 01, 2019 USD ($) $ / £ | Apr. 01, 2019 GBP (£) £ / shares $ / £ | Sep. 20, 2018 GBP (£) | Mar. 31, 2020 USD ($) acquisition | Sep. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2019 acquisition | Jun. 30, 2020 | Feb. 29, 2020 | Jan. 01, 2019
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total consideration |  |  |  |  | $ 245 |  | $ 177 |  |  |  |  |
Cash paid |  |  |  |  | 201 |  | 142 |  |  |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  | 44 |  | 35 |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  | 25 |  | 23 |  |  |  |  |
Contingent consideration from prior year's acquisitions |  |  |  |  | 4 |  | 6 |  |  |  |  |
Assumption of debt |  |  |  |  | 5 |  | 5 |  |  |  |  |
Payments to acquire outstanding minority interests of subsidiaries |  |  |  |  | 0 |  | 88 | $ 79 |  |  |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  | 25 |  | 23 |  |  |  |  |
Contingent consideration from prior year's acquisitions |  |  |  |  | 13 |  | 35 |  |  |  |  |
Jardine Lloyd Thompson Group plc |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total consideration | £ |  |  |  | £ 5.2 |  |  |  |  |  |  |  |
Business acquisition, share price (in gbp per share) | £ / shares |  |  | £ 19.15 |  |  |  |  |  |  |  |  |
Equity interests issued and issuable |  | $ 5,600 | £ 4.3 |  |  |  |  |  |  |  |  |
Business combination, exchange rate | $ / £ |  | 1.31 | 1.31 |  |  |  |  |  |  |  |  |
Assumption of debt |  | $ 1,000 |  |  |  |  |  |  |  |  |  |
Current Fiscal Period Acquisitions |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total consideration |  |  |  |  | 245 |  |  |  |  |  |  |
Cash paid |  |  |  |  | 201 |  |  |  |  |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  | 44 |  |  |  |  |  |  |
Revenue related to acquisitions |  |  |  |  | 15 |  | 4 |  |  |  |  |
Operating (loss) income related to acquisitions |  |  |  |  | $ 4 |  | $ 1 |  |  |  |  |
Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  | 2 |  |  |  | 5 |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  | 2 years |  |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Revenue target period (in years) |  |  |  |  | 4 years |  |  |  |  |  |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of equity method investment sold |  |  |  |  |  |  |  |  |  |  | 4.00% |
Percentage of ownership in equity investment |  |  |  |  | 31.00% |  |  |  |  |  |  |
Marsh India |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership in equity investment |  |  |  |  |  |  |  | 26.00% | 26.00% |  |  | 49.00%
Specialty Business and Large Market Health and Defined Benefit Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Disposal group, consideration |  |  |  |  |  | $ 60 |  |  |  |  |  |
JLT’s Global Aerospace Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Disposal group, consideration | $ 165 |  |  |  |  |  |  |  |  |  |  |
Contingent consideration receivable | $ 65 |  |  |  |  |  |  |  |  |  |  |
Forecast | Alexander Forbes Group Holdings Limited |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of equity method investment sold |  |  |  |  |  |  |  |  |  | 15.00% |  |

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