Supplemental Disclosures (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2019
Supplemental Cash Flow Information [Abstract]
Deferred and contingent purchase consideration from prior years' acquisition	$ 122	$ 65	$ 117
Deferred purchase consideration from prior years' acquisitions	68	43	62
Contingent purchase consideration from prior years' acquisitions	54	22	55
Net charge for adjustments related to acquisitions	26	68	32
Payment of contingent consideration	48	41	36
Non-cash issuances of common stock	219	165	130
Stock-based compensation expense	290	252	$ 193
ROU asset	1,894	1,921		$ 1,700
Lease liability	$ 1,924	$ 1,926		$ 1,900