Acquisitions and Dispositions (Narrative) (Details) $ in Millions		3 Months Ended		12 Months Ended
	Apr. 01, 2022 USD ($)	Mar. 31, 2022 USD ($) acquisition	Mar. 31, 2021 USD ($) disposition acquisition	Dec. 31, 2021 acquisition	Nov. 30, 2021
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition			0
Total consideration		$ 28
Cash paid		24
Estimated fair value of deferred/contingent consideration		4	$ 0
Deferred purchase consideration from prior years' acquisitions		12	27
Deconsolidation of Russian businesses		(39)	$ 0
Number of dispositions | disposition			0
Certain Businesses Primarily in Brazil
Business Acquisition [Line Items]
Disposal group, consideration		4
U.S. Affinity Business | Subsequent Event
Business Acquisition [Line Items]
Disposal group, consideration	$ 145
Gain on disposal	$ 110
Marsh India Insurance Brokers Limited
Business Acquisition [Line Items]
Percentage of ownership in equity investment				92.00%	49.00%
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Estimated fair value of deferred/contingent consideration		4
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration		28
Cash paid		24
Estimated fair value of deferred/contingent consideration		$ 4
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		1		8
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition		2		1
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)		2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)		4 years