Acquisitions and Dispositions of Businesses - Acquisitions (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) acquisition	Dec. 31, 2017 USD ($) acquisition
Business Acquisition
Number of business acquired under business combination | acquisition	3	17
Assets acquired:
Goodwill	$ 8,550	$ 8,358
2018 Acquisitions
Business Combination, Consideration Transferred [Abstract]
Cash	26
Deferred and contingent consideration	3
Aggregate consideration transferred	29
Assets acquired:
Receivables, net	2
Goodwill	12
Intangible assets, net	16
Other assets	3
Total assets acquired	33
Liabilities assumed:
Current liabilities	4
Total liabilities assumed	4
Net assets acquired	$ 29