Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 93	$ 92
Amounts Accrued	9	28
Cash Paid	(29)	(28)
Other	(2)	1
Liability at end of period	71	93
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	15	7
Amounts Accrued	5	17
Cash Paid	(16)	(7)
Other	0	(2)
Liability at end of period	4	15
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	78	85
Amounts Accrued	4	11
Cash Paid	(13)	(21)
Other	(2)	3
Liability at end of period	67	$ 78
Operating Segments | Risk and Insurance Services Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	2
Operating Segments | Consulting Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	1
Corporate, Non-Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	$ 6