Acquisitions and Dispositions of Businesses - Acquisitions (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) acquisition	Jun. 30, 2015 USD ($) acquisition	Jun. 30, 2016 USD ($) acquisition	Jun. 30, 2015 USD ($) acquisition	Dec. 31, 2015 USD ($)
Business Acquisition
Number of business acquired under business combination | acquisition	2	2	4	4
Intangible assets:
Goodwill	$ 8,473		$ 8,473		$ 8,448
Business Acquisition 1 [Member]
Business Acquisition
Consideration	190	$ 23	190	$ 23
Intangible assets:
Goodwill	110	16	110	16
Other intangible assets	82	1	82	1
Total	$ 192	$ 17	$ 192	$ 17
Risk Solutions [Member]
Business Acquisition
Number of business acquired under business combination | acquisition	2	1	2	2
Intangible assets:
Goodwill	$ 5,654		$ 5,654		5,593
HR Solutions [Member]
Business Acquisition
Number of business acquired under business combination | acquisition	0	1		2
Intangible assets:
Goodwill	$ 2,819		$ 2,819		$ 2,855