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Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue | $ 10,770 | $ 10,770 | $ 9,998 | $ 9,409
Operating Segments | United States |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue |  | 4,677 | 4,425 | 3,981
Operating Segments | Americas other than U.S. |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue |  | 940 | 976 | 899
Operating Segments | United Kingdom |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue |  | 1,555 | 1,436 | 1,354
Operating Segments | Europe, Middle East, & Africa other than United Kingdom |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue |  | 2,413 | 2,025 | 1,760
Operating Segments | Asia Pacific |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue |  | $ 1,185 | 1,136 | 1,415
Operating Segments | Commercial Risk Solutions |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue | 4,652 |  | 4,169 | 3,929
Operating Segments | Reinsurance Solutions |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue | 1,563 |  | 1,429 | 1,361
Operating Segments | Retirement Solutions |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue | 1,865 |  | 1,755 | 1,707
Operating Segments | Health Solutions |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue | 1,596 |  | 1,515 | 1,370
Operating Segments | Data & Analytic Services |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue | 1,105 |  | 1,140 | 1,050
Intersegment elimination |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |
Total revenue | $ (11) |  | $ (10) | $ (8)

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- Definition Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045 + Details
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