Acquisitions and Dispositions of Businesses - Dispositions (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) disposal	Jun. 30, 2015 USD ($) disposal	Jun. 30, 2016 USD ($) disposal	Jun. 30, 2015 USD ($) disposal
Dispositions
Gain on disposal of business | $	$ 6	$ 1	$ 41	$ 20
Disposal Group, Not Discontinued Operations [Member]
Dispositions
Number of dispositions	2	0	4	2
Gain on disposal of business | $	$ 6	$ (1)
Disposal Group, Not Discontinued Operations [Member] | Risk Solutions [Member]
Dispositions
Number of dispositions	2	0	3	1
Disposal Group, Not Discontinued Operations [Member] | HR Solutions [Member]
Dispositions
Number of dispositions	0	0	1	1