Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 65	$ 93
Amounts Accrued	6	40
Cash Paid	(8)	(71)
Other	0	3
Liability at end of period	63	65
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	15	32
Amounts Accrued	5	31
Cash Paid	(4)	(49)
Other	(1)	1
Liability at end of period	15	15
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	50	61
Amounts Accrued	1	9
Cash Paid	(4)	(22)
Other	1	2
Liability at end of period	48	$ 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	$ 2