Segment Information - Schedule of total revenue by business segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenue	$ 3,299	$ 2,880	$ 2,919	$ 2,947	$ 3,209	$ 2,794	$ 2,897	$ 2,915	$ 12,045	$ 11,815	$ 11,514
Total operating segments [Member]
Segment Reporting Information [Line Items]
Revenue									12,045	11,815	11,514
Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenue									7,834	7,789	7,632
HR Solutions [Member]
Segment Reporting Information [Line Items]
Revenue									4,264	4,057	3,925
Intersegment elimination [Member]
Segment Reporting Information [Line Items]
Revenue									$ (53)	$ (31)	$ (43)