Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Restructuring Reserve [Roll Forward]
Liability at beginning of period		$ 65	$ 93
Amounts Accrued		95	40
Cash Paid		(80)	(71)
Other		3	3
Liability at end of period	$ 83	83	65
Severance
Restructuring Cost and Reserve [Line Items]
Restructuring costs	84
Restructuring Reserve [Roll Forward]
Liability at beginning of period		15	32
Amounts Accrued		78	31
Cash Paid		(52)	(49)
Other		2	1
Liability at end of period	43	43	15
Severance and Future Rent Under Non-Cancelable Leases and Other Costs [Member]
Restructuring Reserve [Roll Forward]
Amounts Accrued		11
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period		50	61
Amounts Accrued		17	9
Cash Paid		(28)	(22)
Other		1	2
Liability at end of period	$ 40	40	$ 50
Operating Segments | Risk and Insurance Services Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued		3
Operating Segments | Consulting Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued		1
Corporate | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued		$ 7