Revenue - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percentage of total revenue	1.00%
Contract with customer, asset, additions	$ 128
Contract with customer, asset, reclassified to receivable	42
Contract with customer, liability, revenue recognized	169
Performance obligation satisfied in previous period	17
Marsh Insurance Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation	$ 30
Mercer Consulting Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)	30 days
Revenue, remaining performance obligation	$ 429
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	65.00%
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