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Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | $ 11,066 | $ 11,013 | $ 10,770
U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 5,032 | 5,016 | 4,677
Americas other than U.S. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 911 | 919 | 940
U.K. |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,579 | 1,502 | 1,555
Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 84 | 75 | 80
Europe, Middle East, & Africa other than U.K. and Ireland |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 2,236 | 2,263 | 2,333
Asia Pacific |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,224 | 1,238 | 1,185
Commercial Risk Solutions |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 4,690 | 4,673 | 4,652
Reinsurance Solutions |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,814 | 1,686 | 1,563
Retirement Solutions |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,753 | 1,817 | 1,865
Health Solutions |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,655 | 1,667 | 1,596
Data & Analytic Services |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | 1,171 | 1,184 | 1,105
Elimination |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Total revenue | $ (17) | $ (14) | $ (11)

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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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 30 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 40 -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9031-108599 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 41 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9038-108599 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 32 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599 Reference 5: 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=120311839&loc=d3e8933-108599 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130543-203045 Reference 7: 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=120311839&loc=d3e8736-108599 Reference 8: 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=120311839&loc=d3e8736-108599 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 42 -URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599 Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130545-203045 Reference 11: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 924 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 11.L) -URI http://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856 + Details
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