Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 102	$ 112
Amounts Accrued	9	112
Cash Paid	(38)	(112)
Other	(5)	(10)
Liability at end of period	68	102
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	51	73
Amounts Accrued	5	73
Cash Paid	(31)	(91)
Other	(1)	(4)
Liability at end of period	24	51
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	51	39
Amounts Accrued	4	39
Cash Paid	(7)	(21)
Other	(4)	(6)
Liability at end of period	44	51
Jardine Lloyd Thompson Group plc
Restructuring Reserve [Roll Forward]
Liability at beginning of period	47	0
Amounts Accrued	80	335
Cash Paid	(61)	(265)
Non-cash charges	(13)	(23)
Liability at end of period	53	47
Jardine Lloyd Thompson Group plc | Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	42	0
Amounts Accrued	23	154
Cash Paid	(24)	(112)
Non-cash charges	0	0
Liability at end of period	41	42
Jardine Lloyd Thompson Group plc | Real estate related costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	5	0
Amounts Accrued	16	38
Cash Paid	(4)	(14)
Non-cash charges	(12)	(19)
Liability at end of period	5	5
Jardine Lloyd Thompson Group plc | Information Technology
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0	0
Amounts Accrued	21	45
Cash Paid	(17)	(45)
Non-cash charges	0	0
Liability at end of period	4	0
Jardine Lloyd Thompson Group plc | Consulting and other outside services
Restructuring Reserve [Roll Forward]
Liability at beginning of period	0	0
Amounts Accrued	20	98
Cash Paid	(16)	(94)
Non-cash charges	(1)	(4)
Liability at end of period	$ 3	$ 0