Revenue - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) installment	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Disaggregation of Revenue [Line Items]
Percentage of total revenue	1.00%
Number of installments for brokerage revenue | installment	4
Contract with customer, performance obligation satisfied in previous period	$ 71	$ 83	$ 84
Capitalized contract cost, amortization	1,800	1,600	$ 1,500
Deferred Implementation Costs
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	10	19
Costs to Obtain
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	362	328
Costs to Fulfill
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	$ 370	$ 320
Mercer
Disaggregation of Revenue [Line Items]
Consulting revenue, payment terms (in days)	30 days
Minimum
Disaggregation of Revenue [Line Items]
Capitalized contract cost, amortization period	3 months
Maximum
Disaggregation of Revenue [Line Items]
Capitalized contract cost, amortization period	6 months
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
Consulting Segment | Mercer
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, percent	57.00%
Consulting Segment | Minimum | Oliver Wyman Group
Disaggregation of Revenue [Line Items]
Consulting revenue, payment terms (in days)	30 days
Consulting Segment | Maximum | Oliver Wyman Group
Disaggregation of Revenue [Line Items]
Consulting revenue, payment terms (in days)	60 days