Revenue (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) installment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 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	$ 73	$ 71	$ 83
Capitalized contract cost, amortization	1,900	1,800	$ 1,600
Deferred Implementation Costs
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	3	10
Costs to Obtain
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	396	362
Costs to Fulfill
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	$ 397	$ 370
Mercer Consulting Group
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 Consulting Group
Disaggregation of Revenue [Line Items]
Revenue from contract with customer, percent	54.00%
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