Segment Information (Details) $ in Millions	2 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2017 segment	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment revenue_line	Jun. 30, 2016 USD ($)
Segment Reporting Information
Number of reportable segments | segment				1
Number of revenue lines | revenue_line				5
Number of operating segments | segment	1			1
Total revenue		$ 2,368	$ 2,282	$ 4,749	$ 4,558
Intersegment Eliminations
Segment Reporting Information
Total revenue		(4)	(1)	(4)	(3)
Commercial Risk Solutions | Operating Segments
Segment Reporting Information
Total revenue		1,042	990	2,026	1,951
Reinsurance Solutions | Operating Segments
Segment Reporting Information
Total revenue		344	332	715	703
Retirement Solutions | Operating Segments
Segment Reporting Information
Total revenue		389	405	775	800
Health Solutions | Operating Segments
Segment Reporting Information
Total revenue		312	281	684	573
Data & Analytic Services | Operating Segments
Segment Reporting Information
Total revenue		$ 285	$ 275	$ 553	$ 534