Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 112	$ 65
Amounts Accrued	38	161
Cash Paid	(41)	(114)
Other	2	0
Liability at end of period	111	112
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	73	15
Amounts Accrued	37	137
Cash Paid	(37)	(77)
Other	0	(2)
Liability at end of period	73	73
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	39	50
Amounts Accrued	1	24
Cash Paid	(4)	(37)
Other	2	2
Liability at end of period	38	$ 39
Risk and Insurance Services Segment | Severance and Consulting Costs
Restructuring Reserve [Roll Forward]
Amounts Accrued	5
Mercer Consulting Group | Severance and Consulting Costs
Restructuring Reserve [Roll Forward]
Amounts Accrued	10
Jardine Lloyd Thompson Group plc | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	20
Jardine Lloyd Thompson Group plc | Risk and Insurance Services Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	19
Corporate, Non-Segment | Severance and Future Rent Under Non-Cancelable Leases and Other Costs [Member]
Restructuring Reserve [Roll Forward]
Amounts Accrued	3
Corporate, Non-Segment | Jardine Lloyd Thompson Group plc | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	$ 1