Supplemental Disclosures to the Consolidated Statements of Cash Flows (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018
Supplemental Cash Flow Information [Abstract]
Deferred and contingent consideration from prior years acquisition		$ 60	$ 106
Deferred purchase consideration from prior years' acquisitions		37	59
Contingent consideration from prior year's acquisitions		23	47
Net charge for adjustments related to acquisition related accounts		26	19
Payment of contingent consideration		35	29
Non-cash issuance of common stock		164	129
Stock-based compensation expense, equity awards		$ 184	$ 146
Right-of-use asset, noncash	$ 1,700
Right-to-use liability, noncash	$ 1,900