Revenue - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) installment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Percentage of total revenue	2.00%
Number of installments for brokerage revenue | installment	4
Cumulative catch-up adjustment to revenue, change in measure of progress	$ 547	$ 311
Contract with customer, asset, reclassified to receivable	493	284
Cash received for performance obligations not yet fulfilled	642
Contract with customer, liability, revenue recognized	539	527
Contract with customer, performance obligation satisfied in previous period	84	97	$ 79
Capitalized contract cost, amortization	1,500	1,300	$ 1,200
Deferred implementation costs
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Capitalized contract cost, net	24	29
Costs to obtain
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Capitalized contract cost, net	290	253
Costs to fulfill
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Capitalized contract cost, net	$ 316	$ 296
Mercer
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)	30 days
Revenue, remaining performance obligation	$ 187
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 | Operating Segments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Gain (loss) on consolidation of business	$ 267
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%