Fiduciary Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Fiduciary Assets and Liabilities [Line Items]
Net uncollected premiums and claims receivable and payable	$ 7,700		$ 7,700		$ 7,000
Operating Segments | Risk and Insurance Services Segment
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds (less than)	9	$ 6	17	$ 12
Operating Segments | Consulting Segment
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds (less than)	$ 1	$ 1	$ 1	$ 1