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Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Disaggregation of Revenue [Line Items] |  |
Total revenue | $ 3,090 | $ 2,381
United States |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,116 |
Americas other than United States |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 237 |
United Kingdom |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 484 |
Europe, Middle East, & Africa other than United Kingdom |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 979 |
Asia Pacific |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 274 |
Operating Segments | Commercial Risk Solutions |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,184 |
Operating Segments | Reinsurance Solutions |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 742 |
Operating Segments | Retirement Solutions |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 424 |
Operating Segments | Health Solutions |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 451 |
Operating Segments | Data & Analytic Services |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 294 |
Intersegment Eliminations |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | $ (5) |

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- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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