Supplemental Disclosures (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Information [Abstract]
Deferred and contingent purchase consideration from prior years' acquisition	$ 117	$ 136	$ 98
Deferred purchase consideration from prior years' acquisitions	62	55	54
Contingent purchase consideration from prior years' acquisitions	55	81	44
Net charge for adjustments related to acquisitions	32	3	9
Payment of contingent consideration	36	27	42
Non-cash issuance of common stock	130	88	73
Stock-based compensation expense, equity awards	$ 193	$ 149	109
Adjustments to additional paid in capital, income tax benefit from share-based compensation			$ 44