Revenue - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) installment	Dec. 31, 2019 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percentage of total revenue	1.00%
Number of installments for brokerage revenue | installment	4
Cumulative catch-up adjustment to revenue, change in measure of progress	$ 311	$ 437
Contract with customer, asset, reclassified to receivable	284	342
Cash received for performance obligations not yet fulfilled	615
Contract with customer, liability, revenue recognized	527	531
Revenue reduction for estimated commission revenue	42
Contract with customer, performance obligation satisfied in previous period	97	79
Capitalized contract cost, amortization	1,300	1,200
Deferred implementation costs
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Capitalized contract cost, net	29	30
Costs to obtain
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Capitalized contract cost, net	253	222
Costs to fulfill
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Capitalized contract cost, net	$ 296	$ 262
Mercer
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)	30 days
Revenue, remaining performance obligation	$ 184
Oliver Wyman Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation	6
Marsh
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation	$ 38
Minimum | Oliver Wyman Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)	30 days
Maximum | Oliver Wyman Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)	60 days
Risk and insurance services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Insurance brokerage commissions, payment terms (in days)	30 days
Risk and insurance services | Minimum
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
Risk and insurance services | Maximum
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
Health brokerage and consulting services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer, percent	60.00%