Segment Information - Schedule of fiduciary investment income by business segment (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]
Fiduciary investment income	$ 6	$ 6	$ 5	$ 5	$ 5	$ 6	$ 5	$ 5	$ 22	$ 21	$ 26
HR Solutions
Segment Reporting Information [Line Items]
Fiduciary investment income									0	0	0
Risk Solutions
Segment Reporting Information [Line Items]
Fiduciary investment income									$ 22	$ 21	$ 26