Segment Information - Schedule of total revenue by business segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total revenue	$ 2,909	$ 2,340	$ 2,368	$ 2,381	$ 2,650	$ 2,201	$ 2,282	$ 2,276	$ 9,998	$ 9,409	$ 9,480
Intersegment elimination
Segment Reporting Information [Line Items]
Total revenue									(10)	(8)	(11)
Commercial Risk Solutions | Operating Segments
Segment Reporting Information [Line Items]
Total revenue									4,169	3,929	4,029
Reinsurance Solutions | Operating Segments
Segment Reporting Information [Line Items]
Total revenue									1,429	1,361	1,358
Retirement Solutions | Operating Segments
Segment Reporting Information [Line Items]
Total revenue									1,755	1,707	1,916
Health Solutions | Operating Segments
Segment Reporting Information [Line Items]
Total revenue									1,515	1,370	1,167
Data & Analytic Services | Operating Segments
Segment Reporting Information [Line Items]
Total revenue									$ 1,140	$ 1,050	$ 1,021