Acquisitions and Dispositions of Businesses - Acquisitions (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) acquisition	Dec. 31, 2016 USD ($) acquisition
Business Acquisition
Number of business acquired under business combination | acquisition	8	8
Assets acquired:
Goodwill	$ 7,888	$ 7,410
2017 Acquisitions
Business Combination, Consideration Transferred [Abstract]
Cash	164
Deferred and contingent consideration	32
Aggregate consideration transferred	196
Assets acquired:
Cash and cash equivalents	7
Receivables, net	11
Goodwill	121
Intangible assets, net	90
Fixed assets, net	1
Other assets	10
Total assets acquired	240
Liabilities assumed:
Current liabilities	18
Other non-current liabilities	26
Total liabilities assumed	44
Net assets acquired	$ 196