Acquisitions and Dispositions of Businesses - Acquisitions (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) acquisition	Mar. 31, 2015 USD ($) acquisition	Dec. 31, 2015 USD ($)
Business Acquisition
Number of business acquired under business combination | acquisition	2	2
Intangible assets:
Goodwill	$ 8,411		$ 8,448
Business Acquisition 1 [Member]
Business Acquisition
Consideration	21	$ 21
Intangible assets:
Goodwill	7	16
Other intangible assets	8	1
Total	$ 15	$ 17
Risk Solutions [Member]
Business Acquisition
Number of business acquired under business combination | acquisition	0	1
Intangible assets:
Goodwill	$ 5,580		5,593
HR Solutions [Member]
Business Acquisition
Number of business acquired under business combination | acquisition	2	1
Intangible assets:
Goodwill	$ 2,831		$ 2,855