Revenue - Narrative (Details) - USD ($) $ in Millions		6 Months Ended
	Apr. 01, 2019	Jun. 30, 2019
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		$ 282
Transferred to receivables from contract assets recognized at the beginning of the period		176
Cash received for performance obligations not yet fulfilled		362
Revenue recognized that was included in the contract liability balance at the beginning of the period		307
Performance obligation satisfied in previous period		36
Marsh Insurance Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation		$ 22
Mercer Consulting Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)		30 days
Revenue, remaining performance obligation		$ 323
Oliver Wyman Group Consulting Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation		$ 3
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		62.00%
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	$ 62
Increase for contract acquired in business combination	$ 47