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Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2016 | Jun. 30, 2018 | Dec. 31, 2017
Restructuring Reserve [Roll Forward] |  |  |
Liability at beginning of period |  | $ 65 | $ 93
Amounts Accrued |  | 64 | 40
Cash Paid |  | (41) | (71)
Other |  | (1) | 3
Liability at end of period |  | 87 | 65
Severance and Future rent under non-cancelable leases and other costs |  |  |
Restructuring Reserve [Roll Forward] |  |  |
Amounts Accrued |  | 9 |
Severance |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Restructuring costs | $ 55 |  |
Restructuring Reserve [Roll Forward] |  |  |
Liability at beginning of period |  | 15 | 32
Amounts Accrued |  | 51 | 31
Cash Paid |  | (22) | (49)
Other |  | (1) | 1
Liability at end of period |  | 43 | 15
Future rent under non-cancelable leases and other costs |  |  |
Restructuring Reserve [Roll Forward] |  |  |
Liability at beginning of period |  | 50 | 61
Amounts Accrued |  | 13 | 9
Cash Paid |  | (19) | (22)
Other |  | 0 | 2
Liability at end of period |  | 44 | $ 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 |  | $ 5 |

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- Definition Carrying amount (including both current and noncurrent portions of the accrual) as of the balance sheet date pertaining to a specified type of cost associated with exit from or disposal of business activities or restructuring pursuant to a duly authorized plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 5.P.4(b)) -URI http://asc.fasb.org/extlink&oid=115931487&loc=d3e140904-122747 + Details
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