Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 93	$ 93
Amounts Accrued	24	44
Cash Paid	(39)	(39)
Other	(4)	(5)
Liability at end of period	74	93
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	32	15
Amounts Accrued	19	40
Cash Paid	(30)	(22)
Other	(2)	(1)
Liability at end of period	19	32
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	61	78
Amounts Accrued	5	4
Cash Paid	(9)	(17)
Other	(2)	(4)
Liability at end of period	55	$ 61
Operating Segments | Risk and Insurance Services Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	4
Operating Segments | Consulting Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	16
Corporate | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	$ 4