Segment Information - Schedule of total revenue by business segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenue	$ 3,288	$ 2,742	$ 2,805	$ 2,847	$ 3,299	$ 2,880	$ 2,919	$ 2,947	$ 11,682	$ 12,045	$ 11,815
Commissions, fees and other	$ 3,283	$ 2,736	$ 2,800	$ 2,842	$ 3,292	$ 2,873	$ 2,913	$ 2,941	11,661	12,019	11,787
Operating Segments [Member] | Risk Solutions [Member]
Segment Reporting Information [Line Items]
Revenue									7,426	7,834	7,789
Commissions, fees and other									7,405	7,808	7,761
Operating Segments [Member] | HR Solutions [Member]
Segment Reporting Information [Line Items]
Revenue									4,303	4,264	4,057
Commissions, fees and other									4,303	4,264	4,057
Intersegment elimination [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									$ (47)	$ (53)	$ (31)