Acquisitions and Dispositions of Businesses - Acquisitions (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) acquisition	Dec. 31, 2015 USD ($) acquisition	Dec. 31, 2014 USD ($)
Business Acquisition
Number of business acquired under business combination | acquisition	8	7
Goodwill	$ 8,747	$ 8,448	$ 8,860
Risk Solutions
Business Acquisition
Number of business acquired under business combination | acquisition	5	4
Goodwill	$ 5,953	$ 5,593	5,911
HR Solutions
Business Acquisition
Number of business acquired under business combination | acquisition	3	3
Goodwill	$ 2,794	$ 2,855	$ 2,949
2016 Acquisitions
Business Acquisition
Cash	891
Deferred and contingent consideration	43
Aggregate consideration transferred	934
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
Current liabilities	163
Other liabilities	7
Total liabilities assumed	170
Net assets acquired	$ 934
Acquired intangible assets, weighted average useful life (in years)	13 years
Acquisition costs	$ 8
Revenue	$ 68
2015 Acquisitions
Business Acquisition
Aggregate consideration transferred		27
Goodwill		18
Intangible assets, net		6
Total intangible assets		$ 24