Acquisitions and Dispositions of Businesses - Dispositions (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) disposal	Mar. 31, 2015 USD ($) disposal
Dispositions
Gain on disposal of business | $	$ 35	$ 19
Disposal Group, Not Discontinued Operations [Member]
Dispositions
Number of dispositions	2	2
Gain on disposal of business | $	$ 35	$ (19)
Disposal Group, Not Discontinued Operations [Member] | Risk Solutions [Member]
Dispositions
Number of dispositions	1	1
Disposal Group, Not Discontinued Operations [Member] | HR Solutions [Member]
Dispositions
Number of dispositions	1	1