Segment Information - Schedule of total revenue by business segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenue	$ 3,262	$ 2,760	$ 2,800	$ 2,805	$ 3,227	$ 2,752	$ 2,836	$ 2,867	$ 11,627	$ 11,682	$ 12,045
Commissions, fees and other	$ 3,256	$ 2,754	$ 2,795	$ 2,800	$ 3,222	$ 2,746	$ 2,831	$ 2,862	11,605	11,661	12,019
Operating Segments | Risk Solutions
Segment Reporting Information [Line Items]
Revenue									7,485	7,426	7,834
Commissions, fees and other									7,463	7,405	7,808
Operating Segments | HR Solutions
Segment Reporting Information [Line Items]
Revenue									4,183	4,303	4,264
Commissions, fees and other									4,183	4,303	4,264
Intersegment elimination
Segment Reporting Information [Line Items]
Revenue									(41)	(47)	(53)
Commissions, fees and other									$ (41)	$ (47)	$ (53)