Acquisitions and Dispositions (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) acquisition	Sep. 30, 2020 USD ($)	Dec. 31, 2020 acquisition
Business Acquisition [Line Items]
Total consideration			$ 546	$ 742
Cash paid			405	579
Estimated fair value of deferred/contingent consideration			141	163
Deferred purchase consideration from prior years' acquisitions			84	60
Contingent consideration from prior year's acquisitions			26	65
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration			546
Cash paid			405
Estimated fair value of deferred/contingent consideration			141
Revenue related to acquisitions	$ 38		73
Operating income (loss) related to acquisitions	$ 3		4
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Estimated fair value of deferred/contingent consideration			$ 72
Deferred purchase consideration from prior years' acquisitions				60
Revenue related to acquisitions		$ 52		115
Operating income (loss) related to acquisitions		$ (8)		3
Contingent consideration from prior year's acquisitions				101
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition			3		7
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)			2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)			4 years
Certain Businesses Primarily in the U.S., U.K. and Canada
Business Acquisition [Line Items]
Disposal group, consideration			$ 84	$ 93
Gain on disposal			$ 50