Integration and Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs	$ 44	$ 28	$ 12
Liability balance	93	93	92	$ 124
Payments for restructuring	39	$ 28	$ 44
Operating Segments | Risk and Insurance Services Segment | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs	3
Operating Segments | Consulting Segment | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs	34
Corporate, Non-Segment | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs	$ 7