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Revenue - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Revenue - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2020	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue recognized that was included in the contract liability balance at the beginning of the period		$ 418	$ 307
Performance obligation satisfied in previous period		67	$ 36
Revenue reduction for estimated commission revenue	$ 36
Marsh Insurance Group
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation	51	51
Mercer Consulting Group
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation	220	220
Oliver Wyman Group Consulting Group
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation	2	$ 2
Risk and Insurance Services Segment \| Marsh Insurance Group
Disaggregation of Revenue [Line Items]
Revenue reduction for estimated commission revenue	33
Consulting Segment \| Mercer Consulting Group \| Health
Disaggregation of Revenue [Line Items]
Revenue reduction for estimated commission revenue	$ 3