Acquisitions and Dispositions (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) acquisition	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) acquisition
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions	$ 27	$ 25
Contingent consideration from prior year's acquisitions	10	4
Disposal group, consideration	0
Total consideration		245
Cash paid		201
Contingent and deferred consideration	0	44
Revenue related to acquisitions		15
Operating (loss) income related to acquisitions		4
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions	27	25
Contingent consideration from prior year's acquisitions	$ 11	$ 13
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition	0		7
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)	2 years		2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)	4 years		4 years
Certain Businesses Primarily in the U.S., U.K. and Canada
Business Acquisition [Line Items]
Disposal group, consideration			$ 98