Acquisitions (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) acquisition	Mar. 31, 2017 USD ($)	Dec. 31, 2017 acquisition
Business Acquisition [Line Items]
Total consideration	$ 36	$ 509
Cash	29	419
Estimated fair value of deferred/contingent consideration	7	90
Contingent consideration from prior year's acquisitions	30	8
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration	36
Cash	29
Estimated fair value of deferred/contingent consideration	7
Revenue related to acquisitions	3	28
Operating income related to acquisitions	(1)	10
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions	40	26
Contingent consideration from prior year's acquisitions	$ 40	$ 12
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition	2		7
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition	2		3
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