Segment Information - Schedule of reconciliation of segment income before tax to income from continuing operations before income taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes	$ 717	$ 413	$ 277	$ 441	$ 635	$ 417	$ 445	$ 469	$ 1,848	$ 1,966	$ 1,671
Interest income									14	10	9
Interest expense									(273)	(255)	(210)
Other income									100	44	68
Income before income taxes									1,689	1,765	1,538
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									2,042	2,133	1,858
Operating Segments [Member] | Risk Solutions [Member]
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									1,506	1,648	1,540
Operating Segments [Member] | HR Solutions [Member]
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									536	485	318
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									$ (194)	$ (167)	$ (187)