Acquisitions and Dispositions (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) acquisition	Jun. 30, 2020 USD ($)	Dec. 31, 2020 acquisition
Business Acquisition [Line Items]
Total consideration			$ 505	$ 737
Cash paid			367	576
Deferred purchase consideration from prior years' acquisitions			84	27
Estimated fair value of deferred/contingent consideration			138	161
Contingent consideration from prior year's acquisitions			13	42
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration			505
Cash paid			367
Estimated fair value of deferred/contingent consideration			138
Revenue related to acquisitions	$ 35		35
Operating income related to acquisitions	$ 2		2
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions				27
Estimated fair value of deferred/contingent consideration			$ 17
Revenue related to acquisitions		$ 48		63
Operating income related to acquisitions		$ 7		11
Contingent consideration from prior year's acquisitions				78
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition			2		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			$ 81	$ 93
Gain on disposal			$ 50