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Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2018 | Dec. 31, 2017
Restructuring Reserve [Roll Forward] |  |  |
Liability at beginning of period |  | $ 65 | $ 93
Amounts Accrued |  | 95 | 40
Cash Paid |  | (80) | (71)
Other |  | 3 | 3
Liability at end of period | $ 83 | 83 | 65
Severance |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Restructuring costs | 84 |  |
Restructuring Reserve [Roll Forward] |  |  |
Liability at beginning of period |  | 15 | 32
Amounts Accrued |  | 78 | 31
Cash Paid |  | (52) | (49)
Other |  | 2 | 1
Liability at end of period | 43 | 43 | 15
Severance and Future Rent Under Non-Cancelable Leases and Other Costs [Member] |  |  |
Restructuring Reserve [Roll Forward] |  |  |
Amounts Accrued |  | 11 |
Future rent under non-cancelable leases and other costs |  |  |
Restructuring Reserve [Roll Forward] |  |  |
Liability at beginning of period |  | 50 | 61
Amounts Accrued |  | 17 | 9
Cash Paid |  | (28) | (22)
Other |  | 1 | 2
Liability at end of period | $ 40 | 40 | $ 50
Operating Segments | Risk and Insurance Services Segment | Acquisition Related |  |  |
Restructuring Reserve [Roll Forward] |  |  |
Amounts Accrued |  | 3 |
Operating Segments | Consulting Segment | Acquisition Related |  |  |
Restructuring Reserve [Roll Forward] |  |  |
Amounts Accrued |  | 1 |
Corporate | Acquisition Related |  |  |
Restructuring Reserve [Roll Forward] |  |  |
Amounts Accrued |  | $ 7 |

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