Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 81	$ 102
Amounts Accrued	35	89
Cash Paid	(66)	(100)
Non-Cash/Other	(4)	(10)
Liability at end of period	46	81
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	36	51
Amounts Accrued	13	39
Cash Paid	(35)	(54)
Non-Cash/Other	0	0
Liability at end of period	14	36
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	45	51
Amounts Accrued	22	50
Cash Paid	(31)	(46)
Non-Cash/Other	(4)	(10)
Liability at end of period	32	45
Jardine Lloyd Thompson Group plc
Restructuring Reserve [Roll Forward]
Liability at beginning of period	25	47
Amounts Accrued	61	251
Cash Paid	(58)	(226)
Non-cash charges	(13)	(47)
Liability at end of period	15	25
Jardine Lloyd Thompson Group plc | Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	16	42
Amounts Accrued	9	43
Cash Paid	(15)	(69)
Non-cash charges	0	0
Liability at end of period	10	16
Jardine Lloyd Thompson Group plc | Real estate related costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	7	5
Amounts Accrued	19	69
Cash Paid	(9)	(25)
Non-cash charges	(13)	(42)
Liability at end of period	4	7
Jardine Lloyd Thompson Group plc | Information Technology
Restructuring Reserve [Roll Forward]
Liability at beginning of period	2	0
Amounts Accrued	15	62
Cash Paid	(16)	(55)
Non-cash charges	0	(5)
Liability at end of period	1	2
Jardine Lloyd Thompson Group plc | Consulting and other outside services
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0	0
Amounts Accrued	18	77
Cash Paid	(18)	(77)
Non-cash charges	0	0
Liability at end of period	$ 0	$ 0