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Acquisitions / Dispositions (Narrative) (Details) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 USD ($) | Dec. 31, 2017 USD ($) acquisition | Dec. 31, 2016 USD ($) acquisition | Dec. 31, 2015 USD ($)
Business Acquisition [Line Items] |  |  |  |
Total consideration |  | $ 777 | $ 901 |
Cash |  |  | 865 |
Estimated fair value of deferred/contingent consideration |  | 109 | 36 | $ 176
Deferred purchase consideration from prior years' acquisitions |  | 55 | 54 | 36
Contingent payments for acquisitions |  | 81 | 44 | 13
Deferred purchase consideration |  | 136 | 98 | 49
Revenue related to acquisitions |  |  | 25 | 124
Net operating income related to acquisitions |  |  | 4 | $ 7
Business Combination, Acquisition Related Costs |  | 3 | 14 |
Gain on disposition of business | $ 37 |  | 6 |
Current Fiscal Period Acquisitions |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Total consideration |  | 777 |  |
Cash |  | 668 |  |
Estimated fair value of deferred/contingent consideration |  | 109 |  |
Revenue related to acquisitions |  | 156 |  |
Net operating income related to acquisitions |  | 19 |  |
Prior Fiscal Periods Acquisitions |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Estimated fair value of deferred/contingent consideration |  | 108 |  |
Deferred purchase consideration from prior years' acquisitions |  |  | 54 |
Contingent payments for acquisitions |  |  | $ 86 |
Deferred purchase consideration |  | $ 55 |  |
Risk and Insurance Services Segment |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of acquisitions made | acquisition |  | 7 | 9 |
Consulting Segment |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of acquisitions made | acquisition |  | 3 | 6 |
Minimum |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Revenue target period (in years) |  | 2 years |  |
Maximum |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Revenue target period (in years) |  | 4 years |  |

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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e1043-128460 + Details
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- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477 + Details
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