Revenue - Narrative (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percentage of total revenue		1.00%
Increases as a result of cumulative catch-up adjustments arising from changes in the estimate of the stage of completion, excluding amounts transferred to receivables during the period		$ 353
Transferred to receivables from contract assets		262
Cash received for performance obligations not yet fulfilled		400
Revenue recognized that was included in the contract liability balance at the beginning of the period		$ 378
Performance obligation satisfied in previous period	$ 67
Mercer Consulting Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)		30 days
Revenue, remaining performance obligation	127	$ 127
Oliver Wyman Group Consulting Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation	2	2
Guy Carpenter Reinsurance Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract with customer, liability, adjustment	17
Marsh Insurance Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation	$ 33	$ 33
Risk and Insurance Services Segment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Insurance brokerage commissions, payment terms (in days)		30 days
Health Brokerage and Consulting
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer, percent		63.00%
Minimum | Oliver Wyman Group Consulting Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)		30 days
Minimum | Risk and Insurance Services Segment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Estimated brokerage revenue, payment terms (in months)		12 months
Reinsurance brokerage revenue, payment terms (in months)		12 months
Maximum | Oliver Wyman Group Consulting Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)		60 days
Maximum | Risk and Insurance Services Segment
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Estimated brokerage revenue, payment terms (in months)		18 months
Reinsurance brokerage revenue, payment terms (in months)		18 months
Jardine Lloyd Thompson Group plc
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Increase for contract acquired in business combination		$ 58
Increase for contract acquired in business combination		$ 45