Acquisitions and Dispositions - Dispositions (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) disposal	Dec. 31, 2014 USD ($) disposal	Dec. 31, 2013 USD ($) disposal
Dispositions
Net gain on disposals of businesses | $	$ 82	$ 24	$ 10
Disposal Group, Not Discontinued Operations [Member]
Dispositions
Number of dispositions	7	2	9
Disposal Group, Not Discontinued Operations [Member] | Risk Solutions [Member]
Dispositions
Number of dispositions	4	2	7
Disposal Group, Not Discontinued Operations [Member] | HR Solutions [Member]
Dispositions
Number of dispositions	3	0	2