Acquisitions (Narrative) (Details) $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) acquisition	Jun. 30, 2016 USD ($)	Dec. 31, 2016 acquisition
Business Acquisition [Line Items]
Total consideration		$ 510	$ 105
Cash		420	79
Estimated fair value of deferred/contingent consideration		90	26
Contingent consideration from prior year's acquisitions		60	24
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration		510
Cash		420
Estimated fair value of deferred/contingent consideration		90
Revenue related to acquisitions	$ 34	63
Operating income related to acquisitions	$ 5	15
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions		37	39
Contingent consideration from prior year's acquisitions		$ 65	$ 50
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		5		9
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition				6
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)		2 years
Minimum | Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Revenue target period (in years)		2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)		4 years
Maximum | Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Revenue target period (in years)		4 years