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, 2015	Dec. 31, 2014
Changes in the net carrying amount of goodwill by operating segment (in millions)
Balance at the beginning of the period	$ 8,860	$ 8,997
Goodwill related to current year acquisitions	18	292
Goodwill related to disposals	(77)	(14)
Goodwill related to prior year acquisitions	0	(8)
Transfer	0	0
Foreign currency translation	(353)	(407)
Balance at the end of the period	8,448	8,860
Risk Solutions [Member]
Changes in the net carrying amount of goodwill by operating segment (in millions)
Balance at the beginning of the period	5,911	6,020
Goodwill related to current year acquisitions	2	287
Goodwill related to disposals	(1)	(14)
Goodwill related to prior year acquisitions	0	(8)
Transfer	0	(2)
Foreign currency translation	(319)	(372)
Balance at the end of the period	5,593	5,911
HR Solutions [Member]
Changes in the net carrying amount of goodwill by operating segment (in millions)
Balance at the beginning of the period	2,949	2,977
Goodwill related to current year acquisitions	16	5
Goodwill related to disposals	(76)	0
Goodwill related to prior year acquisitions	0	0
Transfer	0	2
Foreign currency translation	(34)	(35)
Balance at the end of the period	$ 2,855	$ 2,949