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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes	$ 568	$ 436	$ 439	$ 463	$ 656	$ 423	$ 308	$ 461	$ 1,906	$ 1,848	$ 1,966
Interest income									9	14	10
Interest expense									(282)	(273)	(255)
Other income									36	100	44
Income before income taxes									1,669	1,689	1,765
Operating Segments
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									2,144	2,042	2,133
Operating Segments | Risk Solutions
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									1,587	1,506	1,648
Operating Segments | HR Solutions
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									557	536	485
Segment Reconciling Items
Segment Reporting Information [Line Items]
Operating income from continuing operations before income taxes									$ (238)	$ (194)	$ (167)