Fiduciary Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Fiduciary Assets and Liabilities [Line Items]
Net uncollected premiums and claims receivable and payable	$ 11,400		$ 11,400		$ 8,900
Risk and Insurance Services Segment
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	8	$ 31	40	$ 80
Operating Segments | Risk and Insurance Services Segment
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	8	31	40	80
Operating Segments | Consulting Segment
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	$ 1	$ 1	$ 1	$ 2