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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, 2019 | Mar. 31, 2018
Disaggregation of Revenue [Line Items] |  |
Total revenue | $ 3,143 | $ 3,090
United States |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,161 | 1,116
Americas other than United States |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 226 | 237
United Kingdom |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 452 | 484
Europe, Middle East, & Africa other than United Kingdom |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,009 | 979
Asia Pacific |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 295 | 274
Operating Segments | Commercial Risk Solutions |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 1,118 | 1,184
Operating Segments | Reinsurance Solutions |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 788 | 742
Operating Segments | Retirement Solutions |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 420 | 424
Operating Segments | Health Solutions |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 486 | 451
Operating Segments | Data & Analytic Services |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | 336 | 294
Elimination |  |
Disaggregation of Revenue [Line Items] |  |
Total revenue | $ (5) | $ (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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 32 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 22 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599 + Details
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