Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Jun. 30, 2018	Dec. 31, 2017
Restructuring Reserve [Roll Forward]
Liability at beginning of period		$ 65	$ 93
Amounts Accrued		64	40
Cash Paid		(41)	(71)
Other		(1)	3
Liability at end of period		87	65
Severance and Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Amounts Accrued		9
Severance
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 55
Restructuring Reserve [Roll Forward]
Liability at beginning of period		15	32
Amounts Accrued		51	31
Cash Paid		(22)	(49)
Other		(1)	1
Liability at end of period		43	15
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period		50	61
Amounts Accrued		13	9
Cash Paid		(19)	(22)
Other		0	2
Liability at end of period		44	$ 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		$ 5