Acquisitions (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) acquisition	Mar. 31, 2015 USD ($)	Dec. 31, 2015 acquisition
Business Acquisition [Line Items]
Total consideration	$ 103	$ 32
Cash	77	20
Estimated fair value of deferred/contingent consideration	26	12
Contingent consideration from prior year's acquisitions	14	6
Purchases of other intangible assets		3
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration	103
Cash	77
Estimated fair value of deferred/contingent consideration	26
Revenue related to acquisitions	2
Operating income related to acquisitions	1
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions	25	26
Contingent consideration from prior year's acquisitions	$ 18	$ 19
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	1		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