Acquisitions and Dispositions of Businesses - Acquisitions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets acquired
Goodwill	$ 8,171	$ 8,358	$ 7,410
2018 Acquisitions
Consideration transferred
Cash	55
Deferred and contingent consideration	18
Aggregate consideration transferred	73
Assets acquired
Cash and cash equivalents	1
Receivables, net	4
Goodwill	38
Intangible assets, net	34
Other assets	4
Total assets acquired	81
Liabilities assumed
Current liabilities	6
Other liabilities	2
Total liabilities assumed	8
Net assets acquired	$ 73