Integration and Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs		$ 161	$ 40	$ 44
Liability balance	$ 112	112	65	93	$ 93
Payments for restructuring		114	71	44
Severance
Restructuring Cost and Reserve [Line Items]
Restructuring costs	51	96
Restructuring actions costs		137	31
Liability balance	$ 73	73	15	$ 32
Payments for restructuring		77	$ 49
Severance and Future Rent
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs		14
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		1
Corporate, Non-Segment | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs		$ 10