Acquisitions (Narrative) (Details) $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) acquisition	Sep. 30, 2016 USD ($)	Dec. 31, 2016 acquisition
Business Acquisition [Line Items]
Total consideration		$ 734	$ 119
Cash		640	90
Estimated fair value of deferred/contingent consideration		94	29
Contingent consideration from prior year's acquisitions		80	43
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration		734
Cash		640
Estimated fair value of deferred/contingent consideration		94
Revenue related to acquisitions	$ 40	103
Operating income related to acquisitions	$ (1)	16
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions		47	53
Contingent consideration from prior year's acquisitions		$ 107	$ 85
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		7		9
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		1		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