Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended	30 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2021
Restructuring Reserve [Roll Forward]
Liability at beginning of period		$ 112	$ 65	$ 72
Amounts Accrued		44	161
Cash Paid		(82)	(114)
Other		(2)	0
Liability at end of period	$ 72	72	112
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period		73	15	37
Amounts Accrued		38	137
Cash Paid		(73)	(77)
Other		(1)	(2)
Liability at end of period	37	37	73
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period		39	50	35
Amounts Accrued		6	24
Cash Paid		(9)	(37)
Other		(1)	2
Liability at end of period	35	35	39
Risk and Insurance Services Segment | Severance and Consulting Costs
Restructuring Reserve [Roll Forward]
Amounts Accrued	2	7
Mercer Consulting Group | Severance and Consulting Costs
Restructuring Reserve [Roll Forward]
Amounts Accrued	22	32
Jardine Lloyd Thompson Group plc
Restructuring Reserve [Roll Forward]
Liability at beginning of period		0		74
Amounts Accrued		134
Cash Paid		(60)
Other		0
Liability at end of period	74	74	0
Jardine Lloyd Thompson Group plc | Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period		0		46
Amounts Accrued		73
Cash Paid		(27)
Other		0
Liability at end of period	46	46	0
Jardine Lloyd Thompson Group plc | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	98	134
Jardine Lloyd Thompson Group plc | Real estate related costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period		0		0
Amounts Accrued		0
Cash Paid		0
Other		0
Liability at end of period	0	0	0
Jardine Lloyd Thompson Group plc | Consulting and other outside services
Restructuring Reserve [Roll Forward]
Liability at beginning of period		0		28
Amounts Accrued		61
Cash Paid		(33)
Other		0
Liability at end of period	28	28	$ 0
Jardine Lloyd Thompson Group plc | Risk and Insurance Services Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	75	95
Jardine Lloyd Thompson Group plc | Consulting Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	5	5
Corporate, Non-Segment | Severance and Future Rent Under Non-Cancelable Leases and Other Costs
Restructuring Reserve [Roll Forward]
Amounts Accrued		5
Corporate, Non-Segment | Jardine Lloyd Thompson Group plc | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	$ 18	$ 34
Scenario, Forecast | Jardine Lloyd Thompson Group plc | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued				$ 375