Restructuring Costs - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Restructuring Cost and Reserve [Line Items]
Amounts accrued		$ 20	$ 112
ROU asset impairment loss	$ 11
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		6
Consulting costs and adjustments to restructuring liabilities for future rent under non-cancellable leases | Corporate
Restructuring Cost and Reserve [Line Items]
Amounts accrued		12
Jardine Lloyd Thompson Group plc
Restructuring Cost and Reserve [Line Items]
Amounts accrued		137	335
Jardine Lloyd Thompson Group plc | Acquisition related
Restructuring Cost and Reserve [Line Items]
Restructuring and related cost, expected cost	700	700
Restructuring and related cost, cost incurred to date	$ 137	137	$ 335
Amounts accrued		137
Jardine Lloyd Thompson Group plc | Acquisition related | Corporate
Restructuring Cost and Reserve [Line Items]
Amounts accrued		20
Jardine Lloyd Thompson Group plc | Acquisition related | Risk and Insurance Services Segment
Restructuring Cost and Reserve [Line Items]
Amounts accrued		100
Jardine Lloyd Thompson Group plc | Acquisition related | Consulting Segment
Restructuring Cost and Reserve [Line Items]
Amounts accrued		$ 17