Acquisitions and Dispositions - Dispositions (Details) (Disposal Groups [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 business	Dec. 31, 2013 business	Dec. 31, 2012 business
Risk Solutions [Member]
Dispositions
Number of dispositions	2	7	3
HR Solutions [Member]
Dispositions
Number of dispositions		2	1
Other Income [Member]
Dispositions
Pretax gain recognized on sale of businesses	24	10	1