Acquisitions (Narrative) (Details) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) acquisition	Jun. 30, 2015 USD ($)	Dec. 31, 2015 acquisition
Business Acquisition [Line Items]
Total consideration	$ 105	$ 331
Cash	79	236
Estimated fair value of deferred/contingent consideration	26	95
Contingent consideration from prior year's acquisitions	24	11
Purchases of other intangible assets		3
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration	105
Cash	79
Estimated fair value of deferred/contingent consideration	26
Revenue related to acquisitions	9
Operating income related to acquisitions	2
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions	39	28
Contingent consideration from prior year's acquisitions	$ 50	$ 33
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition	3		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)	3 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