Restructuring Costs - Narrative (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Restructuring Cost and Reserve [Line Items]
Amounts accrued	$ 43	$ 112
ROU asset impairment loss	3
Severance and consulting costs | Marsh Insurance Group
Restructuring Cost and Reserve [Line Items]
Amounts accrued	2
Severance and consulting costs | Mercer Consulting Group
Restructuring Cost and Reserve [Line Items]
Amounts accrued	17
Cost To Modernize IT Systems And Security Protocols, Consulting Costs, And Adjustments To Restructuring Liabilities | Corporate
Restructuring Cost and Reserve [Line Items]
Amounts accrued	24
Jardine Lloyd Thompson Group plc
Restructuring Cost and Reserve [Line Items]
Amounts accrued	181	335
ROU asset impairment loss	9
Jardine Lloyd Thompson Group plc | Acquisition related
Restructuring Cost and Reserve [Line Items]
Restructuring and related cost, expected cost	700
Restructuring and related cost, cost incurred to date	181	$ 335
Amounts accrued	181
Jardine Lloyd Thompson Group plc | Acquisition related | Corporate
Restructuring Cost and Reserve [Line Items]
Amounts accrued	25
Jardine Lloyd Thompson Group plc | Acquisition related | Risk and Insurance Services Segment
Restructuring Cost and Reserve [Line Items]
Amounts accrued	125
Jardine Lloyd Thompson Group plc | Acquisition related | Consulting Segment
Restructuring Cost and Reserve [Line Items]
Amounts accrued	$ 31