Segment Information - Schedule of commissions, fees, and other revenue by products (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]
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]
Commissions, fees and other									7,463	7,405	7,808
Operating Segments | HR Solutions
Segment Reporting Information [Line Items]
Commissions, fees and other									4,183	4,303	4,264
Operating Segments | Reportable Subsegments | Retail Brokerage Revenues | Risk Solutions
Segment Reporting Information [Line Items]
Commissions, fees and other									6,096	6,044	6,334
Operating Segments | Reportable Subsegments | Reinsurance Brokerage Revenues | Risk Solutions
Segment Reporting Information [Line Items]
Commissions, fees and other									1,367	1,361	1,474
Operating Segments | Reportable Subsegments | Consulting Services Revenues | HR Solutions
Segment Reporting Information [Line Items]
Commissions, fees and other									1,662	1,686	1,700
Operating Segments | Reportable Subsegments | Outsourcing Revenues | HR Solutions
Segment Reporting Information [Line Items]
Commissions, fees and other									2,557	2,658	2,607
Operating Segments | Intersubsegment Eliminations | HR Solutions
Segment Reporting Information [Line Items]
Commissions, fees and other									(36)	(41)	(43)
Intersegment elimination
Segment Reporting Information [Line Items]
Commissions, fees and other									$ (41)	$ (47)	$ (53)