Segment Information - Schedule of reconciliation of segment income before tax to income from continuing operations before income taxes (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]
Operating income from continuing operations before income taxes	$ 635	$ 417	$ 445	$ 469	$ 515	$ 364	$ 382	$ 410	$ 1,966	$ 1,671	$ 1,596
Interest income									10	9	10
Interest expense									(255)	(210)	(228)
Other income									44	68	2
Income before income taxes									1,765	1,538	1,380
Risk Solutions [Member]
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									1,648	1,540	1,493
HR Solutions [Member]
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									485	318	289
Total operating segments [Member]
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									2,133	1,858	1,782
Unallocated expense [Member]
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									$ (167)	$ (187)	$ (186)