Fiduciary Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fiduciary Assets and Liabilities [Line Items]
Net uncollected premiums and claims receivable and payable	$ 6,800		$ 6,800		$ 7,000
Operating Segments | Risk and Insurance Services Segment
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds (less than)	11	$ 8	28	$ 20
Operating Segments | Consulting Segment
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds (less than)	$ 2	$ 1	$ 3	$ 2