Supplemental Disclosures (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2019
Supplemental Cash Flow Information [Abstract]
Deferred and contingent purchase consideration from prior years' acquisition	$ 65	$ 117	$ 136
Deferred purchase consideration from prior years' acquisitions	43	62	55
Contingent purchase consideration from prior years' acquisitions	22	55	81
Net charge for adjustments related to acquisitions	68	32	3
Payment of contingent consideration	41	36	27
Non-cash issuances of common stock	165	130	88
Stock-based compensation expense	252	$ 193	$ 149
ROU asset	1,921			$ 1,700
Lease liability	$ 1,926			$ 1,900