Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 93	$ 93
Amounts Accrued	32	44
Cash Paid	(57)	(39)
Other	1	(5)
Liability at end of period	69	93
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	32	15
Amounts Accrued	24	40
Cash Paid	(42)	(22)
Other	0	(1)
Liability at end of period	14	32
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	61	78
Amounts Accrued	8	4
Cash Paid	(15)	(17)
Other	1	(4)
Liability at end of period	55	$ 61
Operating Segments | Risk and Insurance Services Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	7
Operating Segments | Consulting Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	18
Corporate | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	$ 7