Acquisitions and Dispositions of Businesses - Acquisitions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition
Goodwill	$ 8,358	$ 7,410	$ 7,068
2017 Acquisitions
Business Acquisition
Cash	1,136
Deferred and contingent consideration	63
Aggregate consideration transferred	1,199
Cash and cash equivalents	108
Receivables, net	47
Goodwill	619
Intangible assets, net	567
Fixed assets, net	18
Other assets	200
Total assets acquired	1,559
Current liabilities	230
Other liabilities	130
Total liabilities assumed	360
Net assets acquired	$ 1,199