Acquisitions and Dispositions - Acquisitions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014 business	Sep. 30, 2013 business	Sep. 30, 2014 business	Sep. 30, 2013 business	Dec. 31, 2013
Intangible assets:
Goodwill	$ 9,026		$ 9,026		$ 8,997
Business Acquisition 1 [Member]
Business Acquisition
Consideration	446	26	446	26
Intangible assets:
Goodwill	261	26	261	26
Other intangible assets	310	8	310	8
Total	571	34	571	34
Risk Solutions [Member]
Business Acquisition
Number of business acquired under business combination	4	1	7	5
Intangible assets:
Goodwill	6,057		6,057		6,020
HR Solutions [Member]
Business Acquisition
Number of business acquired under business combination	1	1	2	2
Intangible assets:
Goodwill	$ 2,969		$ 2,969		$ 2,977