Segment Information - Schedule of commissions, fees, and other revenue by products (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]
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]
Commissions, fees and other									7,405	7,808	7,761
Operating Segments [Member] | HR Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									4,303	4,264	4,057
Operating Segments [Member] | Reinsurance Brokerage Revenues [Member] | Risk Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									1,361	1,474	1,505
Operating Segments [Member] | Retail Brokerage Revenues [Member] | Risk Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									6,044	6,334	6,256
Operating Segments [Member] | Reportable Subsegments [Member] | Outsourcing Revenues [Member] | HR Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									2,658	2,607	2,469
Operating Segments [Member] | Reportable Subsegments [Member] | Consulting Services Revenues [Member] | HR Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									1,686	1,700	1,626
Operating Segments [Member] | Intersubsegment Eliminations [Member] | HR Solutions [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									(41)	(43)	(38)
Intersegment elimination [Member]
Segment Reporting Information [Line Items]
Commissions, fees and other									$ (47)	$ (53)	$ (31)