Acquisitions and Dispositions of Businesses - Acquisitions (Details) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) acquisition	Dec. 31, 2016 USD ($) acquisition
Business Acquisition
Number of business acquired under business combination | acquisition	4	8
Assets acquired:
Goodwill	$ 7,745	$ 7,410
2017 Acquisitions
Business Combination, Consideration Transferred [Abstract]
Cash	148
Deferred and contingent consideration	28
Aggregate consideration transferred	176
Assets acquired:
Cash and cash equivalents	5
Receivables, net	11
Goodwill	119
Intangible assets, net	69
Fixed assets, net	1
Other assets	8
Total assets acquired	213
Liabilities assumed:
Current liabilities	15
Other liabilities	22
Total liabilities assumed	37
Net assets acquired	$ 176