Acquisitions and Dispositions of Businesses - Dispositions (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) disposal	Sep. 30, 2015 USD ($) disposal	Sep. 30, 2016 USD ($) disposal	Sep. 30, 2015 USD ($) disposal
Dispositions
Gain on disposal of business | $	$ 0	$ 0	$ 41	$ 20
Disposal Group, Not Discontinued Operations [Member]
Dispositions
Number of dispositions	0	1	4	3
Disposal Group, Not Discontinued Operations [Member] | Risk Solutions [Member]
Dispositions
Number of dispositions	0	1	3	2
Disposal Group, Not Discontinued Operations [Member] | HR Solutions [Member]
Dispositions
Number of dispositions	0	0	1	1