Restructuring Costs - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring And related cost, period for expected cost		3 years
Amounts accrued		$ 56	$ 161
Jardine Lloyd Thompson Group plc
Restructuring Cost and Reserve [Line Items]
Amounts accrued		192
Jardine Lloyd Thompson Group plc | Acquisition related
Restructuring Cost and Reserve [Line Items]
Restructuring and related cost, expected cost	$ 375	375
Restructuring and related cost, cost incurred to date	192	192
Amounts accrued	77
Risk and Insurance Services Segment | Severance and consulting costs
Restructuring Cost and Reserve [Line Items]
Amounts accrued		6
Risk and Insurance Services Segment | Jardine Lloyd Thompson Group plc | Acquisition related
Restructuring Cost and Reserve [Line Items]
Amounts accrued	58	134
Consulting Segment | Jardine Lloyd Thompson Group plc | Acquisition related
Restructuring Cost and Reserve [Line Items]
Amounts accrued	5	10
Corporate | Severance and future rent under non-cancelable leases and other costs
Restructuring Cost and Reserve [Line Items]
Amounts accrued		7
Corporate | Jardine Lloyd Thompson Group plc | Acquisition related
Restructuring Cost and Reserve [Line Items]
Amounts accrued	14	48
Mercer Consulting Group | Severance and consulting costs
Restructuring Cost and Reserve [Line Items]
Amounts accrued	$ 10	$ 43