Revenue - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) installment	Dec. 31, 2018 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		$ 437	$ 331
Contract with customer, asset, reclassified to receivable		342	347
Cash received for performance obligations not yet fulfilled		518
Contract with customer, liability, revenue recognized		531	582
Contract with customer, performance obligation satisfied in previous period		79	51
Capitalized contract cost, amortization		$ 1,200	1,000
Risk and insurance services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Insurance brokerage commissions, payment terms (in days)		30 days
Health brokerage and consulting services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer, percent		63.00%
Mercer
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)		30 days
Revenue, remaining performance obligation		$ 104
Oliver Wyman Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation		4
Guy Carpenter
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract with customer, liability, adjustment	$ 17
Marsh
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation		39
Deferred implementation costs
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Capitalized contract cost, net		30	41
Costs to obtain
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Capitalized contract cost, net		222	206
Costs to fulfill
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Capitalized contract cost, net		$ 262	$ 227
Minimum | Risk and insurance services
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
Minimum | Oliver Wyman Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)		30 days
Maximum | Risk and insurance services
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
Maximum | Oliver Wyman Group
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Consulting revenue, payment terms (in days)		60 days
JLT Transaction
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Increase for contract acquired in business combination		$ 42