Acquisitions (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) acquisition	Mar. 31, 2016 USD ($)	Dec. 31, 2016 acquisition
Business Acquisition [Line Items]
Total consideration	$ 509	$ 103
Cash	419	77
Estimated fair value of deferred/contingent consideration	90	26
Contingent consideration from prior year's acquisitions	8	14
Acquisition related expenses (less than)	1
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration	509
Cash	419
Estimated fair value of deferred/contingent consideration	90
Revenue related to acquisitions	28
Operating income related to acquisitions	10
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions	26	25
Contingent consideration from prior year's acquisitions	$ 12	$ 18
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition	4		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