Acquisitions (Narrative) (Details) $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($) acquisition	Sep. 30, 2015 USD ($)	Dec. 31, 2015 acquisition
Business Acquisition [Line Items]
Total consideration		$ 119	$ 602
Cash		90	448
Estimated fair value of deferred/contingent consideration		29	154
Contingent consideration from prior year's acquisitions		43	12
Purchases of other intangible assets			2
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration		119
Cash		90
Estimated fair value of deferred/contingent consideration		29
Revenue related to acquisitions	$ 7	17
Operating income related to acquisitions	$ 2	3
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions		53	30
Contingent consideration from prior year's acquisitions		$ 85	$ 39
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		5		13
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		2		8
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	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	4 years