Revenue - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) installment	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Disaggregation of Revenue [Line Items]
Percentage of total revenue	2.00%
Number of installments for brokerage revenue | installment	4
Contract with customer, performance obligation satisfied in previous period	$ 83	$ 84	$ 97
Capitalized contract cost, amortization	1,600	1,500	$ 1,300
Deferred Implementation Costs
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	19	24
Costs to Obtain
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	328	290
Costs to Fulfill
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	$ 320	$ 316
Mercer
Disaggregation of Revenue [Line Items]
Consulting revenue, payment terms (in days)	30 days
Revenue, remaining performance obligation	$ 231
Minimum | Oliver Wyman Group
Disaggregation of Revenue [Line Items]
Consulting revenue, payment terms (in days)	30 days
Maximum | Oliver Wyman Group
Disaggregation of Revenue [Line Items]
Consulting revenue, payment terms (in days)	60 days
Risk and Insurance Services
Disaggregation of Revenue [Line Items]
Insurance brokerage commissions, payment terms (in days)	30 days
Risk and Insurance Services | Minimum
Disaggregation of Revenue [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
Disaggregation of Revenue [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
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, percent	59.00%