Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 81	$ 102
Amounts Accrued	11	89
Cash Paid	(28)	(100)
Non-Cash/Other	0	(10)
Liability at end of period	64	81
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	36	51
Amounts Accrued	5	39
Cash Paid	(20)	(54)
Non-Cash/Other	0	0
Liability at end of period	21	36
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	45	51
Amounts Accrued	6	50
Cash Paid	(8)	(46)
Non-Cash/Other	0	(10)
Liability at end of period	43	45
Jardine Lloyd Thompson Group plc
Restructuring Reserve [Roll Forward]
Liability at beginning of period	25	47
Amounts Accrued	23	251
Cash Paid	(26)	(226)
Non-cash charges	(4)	(47)
Liability at end of period	18	25
Jardine Lloyd Thompson Group plc | Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	16	42
Amounts Accrued	2	43
Cash Paid	(10)	(69)
Non-cash charges	0	0
Liability at end of period	8	16
Jardine Lloyd Thompson Group plc | Real estate related costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	7	5
Amounts Accrued	6	69
Cash Paid	(2)	(25)
Non-cash charges	(4)	(42)
Liability at end of period	7	7
Jardine Lloyd Thompson Group plc | Information Technology
Restructuring Reserve [Roll Forward]
Liability at beginning of period	2	0
Amounts Accrued	7	62
Cash Paid	(7)	(55)
Non-cash charges	0	(5)
Liability at end of period	2	2
Jardine Lloyd Thompson Group plc | Consulting and other outside services
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0	0
Amounts Accrued	8	77
Cash Paid	(7)	(77)
Non-cash charges	0	0
Liability at end of period	$ 1	$ 0