Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 112	$ 65
Amounts Accrued	56	161
Cash Paid	(108)	(114)
Other	(10)	0
Liability at end of period	50	112
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	73	15
Amounts Accrued	44	137
Cash Paid	(95)	(77)
Other	(4)	(2)
Liability at end of period	18	73
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	39	50
Amounts Accrued	12	24
Cash Paid	(13)	(37)
Other	(6)	2
Liability at end of period	32	39
Jardine Lloyd Thompson Group plc
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0
Amounts Accrued	192
Cash Paid	(143)
Non-cash charges	(7)
Liability at end of period	42	0
Jardine Lloyd Thompson Group plc | Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0
Amounts Accrued	92
Cash Paid	(59)
Non-cash charges	0
Liability at end of period	33	0
Jardine Lloyd Thompson Group plc | Real estate related costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0
Amounts Accrued	21
Cash Paid	(5)
Non-cash charges	(7)
Liability at end of period	9	0
Jardine Lloyd Thompson Group plc | Information Technology
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0
Amounts Accrued	9
Cash Paid	(9)
Non-cash charges	0
Liability at end of period	0	0
Jardine Lloyd Thompson Group plc | Consulting and other outside services
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0
Amounts Accrued	70
Cash Paid	(70)
Non-cash charges	0
Liability at end of period	$ 0	$ 0