Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Restructuring Cost and Reserve [Line Items]
Costs incurred	$ 11		$ 89
Severance and Future Rent
Restructuring Cost and Reserve [Line Items]
Costs incurred	11
Jardine Lloyd Thompson Group plc
Restructuring Cost and Reserve [Line Items]
Costs incurred	23		$ 251
Jardine Lloyd Thompson Group plc | Acquisition related
Restructuring Cost and Reserve [Line Items]
Restructuring and related cost, cost incurred to date	609
Costs incurred	23	$ 80
Jardine Lloyd Thompson Group plc | Acquisition related | Corporate
Restructuring Cost and Reserve [Line Items]
Costs incurred	1
Jardine Lloyd Thompson Group plc | Acquisition related | Risk and Insurance Services Segment
Restructuring Cost and Reserve [Line Items]
Costs incurred	16
Jardine Lloyd Thompson Group plc | Acquisition related | Consulting Segment
Restructuring Cost and Reserve [Line Items]
Costs incurred	$ 6