Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 102	$ 112
Amounts Accrued	43	112
Cash Paid	(85)	(112)
Other	(2)	(10)
Liability at end of period	58	102
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	51	73
Amounts Accrued	15	73
Cash Paid	(51)	(91)
Other	0	(4)
Liability at end of period	15	51
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	51	39
Amounts Accrued	28	39
Cash Paid	(34)	(21)
Other	(2)	(6)
Liability at end of period	43	51
Jardine Lloyd Thompson Group plc
Restructuring Reserve [Roll Forward]
Liability at beginning of period	47	0
Amounts Accrued	181	335
Cash Paid	(169)	(265)
Non-cash charges	(31)	(23)
Liability at end of period	28	47
Jardine Lloyd Thompson Group plc | Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	42	0
Amounts Accrued	38	154
Cash Paid	(70)	(112)
Non-cash charges	0	0
Liability at end of period	10	42
Jardine Lloyd Thompson Group plc | Real estate related costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	5	0
Amounts Accrued	42	38
Cash Paid	(15)	(14)
Non-cash charges	(26)	(19)
Liability at end of period	6	5
Jardine Lloyd Thompson Group plc | Information Technology
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0	0
Amounts Accrued	45	45
Cash Paid	(40)	(45)
Non-cash charges	(5)	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	0
Amounts Accrued	56	98
Cash Paid	(44)	(94)
Non-cash charges	0	(4)
Liability at end of period	$ 12	$ 0