Acquisitions and Dispositions of Businesses - Acquisitions (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) acquisition	Dec. 31, 2016 USD ($) acquisition
Business Acquisition
Number of business acquired under business combination | acquisition	2	8
Assets acquired:
Goodwill	$ 7,544	$ 7,410
2016 Acquisitions
Business Combination, Consideration Transferred [Abstract]
Cash		891
Deferred and contingent consideration		43
Aggregate consideration transferred		934
Assets acquired:
Cash and cash equivalents		12
Receivables, net		52
Goodwill		642
Intangible assets, net		366
Fixed assets, net		30
Other assets		2
Total assets acquired		1,104
Liabilities assumed:
Current liabilities		163
Other liabilities		7
Total liabilities assumed		170
Net assets acquired		$ 934
2017 Acquisitions
Business Combination, Consideration Transferred [Abstract]
Cash	47
Deferred and contingent consideration	5
Aggregate consideration transferred	52
Assets acquired:
Cash and cash equivalents	1
Receivables, net	2
Goodwill	33
Intangible assets, net	23
Fixed assets, net	1
Other assets	1
Total assets acquired	61
Liabilities assumed:
Current liabilities	3
Other liabilities	6
Total liabilities assumed	9
Net assets acquired	$ 52