Segment Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) segment revenue_line	Mar. 31, 2016 USD ($)
Segment Reporting Information
Number of reportable segments | segment	1
Number of revenue lines | revenue_line	5
Number of operating segments | segment	1
Total revenue	$ 2,381	$ 2,276
Intersegment Eliminations
Segment Reporting Information
Total revenue	0	(2)
Commercial Risk Solutions | Operating Segments
Segment Reporting Information
Total revenue	984	961
Reinsurance Solutions | Operating Segments
Segment Reporting Information
Total revenue	371	371
Retirement Solutions | Operating Segments
Segment Reporting Information
Total revenue	386	395
Health Solutions | Operating Segments
Segment Reporting Information
Total revenue	372	292
Data & Analytic Services | Operating Segments
Segment Reporting Information
Total revenue	$ 268	$ 259