Acquisitions and Dispositions - Acquisitions (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) business	Jun. 30, 2014 USD ($) business	Jun. 30, 2015 USD ($) business	Jun. 30, 2014 USD ($) business	Dec. 31, 2014 USD ($)
Intangible assets:
Goodwill	$ 8,661		$ 8,661		$ 8,860
Business Acquisition 1 [Member]
Business Acquisition
Consideration	23	$ 83	23	$ 83
Intangible assets:
Goodwill	16	52	16	52
Other intangible assets	1	48	1	48
Total	$ 17	$ 100	$ 17	$ 100
Risk Solutions [Member]
Business Acquisition
Number of business acquired under business combination | business	1	2	2	3
Intangible assets:
Goodwill	$ 5,722		$ 5,722		5,911
HR Solutions [Member]
Business Acquisition
Number of business acquired under business combination | business	1		2	1
Intangible assets:
Goodwill	$ 2,939		$ 2,939		$ 2,949