Goodwill and Other Intangible Assets - Schedule of changes in the net carrying amount of goodwill by operating segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Changes in the net carrying amount of goodwill by operating segment (in millions)
Balance at the beginning of the period	$ 8,448	$ 8,860
Goodwill related to current year acquisitions	642	18
Goodwill related to disposals	(34)	(77)
Goodwill related to prior year acquisitions	4	0
Foreign currency translation	(313)	(353)
Balance at the end of the period	8,747	8,448
Risk Solutions
Changes in the net carrying amount of goodwill by operating segment (in millions)
Balance at the beginning of the period	5,593	5,911
Goodwill related to current year acquisitions	632	2
Goodwill related to disposals	(8)	(1)
Goodwill related to prior year acquisitions	4	0
Foreign currency translation	(268)	(319)
Balance at the end of the period	5,953	5,593
HR Solutions
Changes in the net carrying amount of goodwill by operating segment (in millions)
Balance at the beginning of the period	2,855	2,949
Goodwill related to current year acquisitions	10	16
Goodwill related to disposals	(26)	(76)
Goodwill related to prior year acquisitions	0	0
Foreign currency translation	(45)	(34)
Balance at the end of the period	$ 2,794	$ 2,855