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Segment Information - Schedule of total revenue by business segments (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue | $ 2,909 | $ 2,340 | $ 2,368 | $ 2,381 | $ 2,650 | $ 2,201 | $ 2,282 | $ 2,276 | $ 9,998 | $ 9,409 | $ 9,480
Intersegment elimination |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | (10) | (8) | (11)
Commercial Risk Solutions | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 4,169 | 3,929 | 4,029
Reinsurance Solutions | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 1,429 | 1,361 | 1,358
Retirement Solutions | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 1,755 | 1,707 | 1,916
Health Solutions | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 1,515 | 1,370 | 1,167
Data & Analytic Services | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | $ 1,140 | $ 1,050 | $ 1,021

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