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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