Fiduciary Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	$ 6	$ 6		$ 18
Net uncollected premiums and claims receivable and payable	7,400		$ 7,400		$ 7,300
Mercer Consulting Group [Member]
Fiduciary Assets and Liabilities [Line Items]
Assets in trusts or funds for management or trustee fee	138,000		138,000
Variable Interest Entity, Not Primary Beneficiary [Member] | Mercer Consulting Group [Member]
Fiduciary Assets and Liabilities [Line Items]
Assets in trusts or funds for management or trustee fee	24,000		24,000
Risk and Insurance Services Segment [Member]
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds			16
Consulting Segment [Member]
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	$ 1	$ 2	$ 3	$ 5