Fiduciary Assets and Liabilities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Fiduciary Assets and Liabilities [Line Items]
Net uncollected premiums and claims receivable and payable	$ 8,900		$ 7,300
Mercer Consulting Group [Member]
Fiduciary Assets and Liabilities [Line Items]
Assets in trusts or funds for management or trustee fee	136,000
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	25,000
Risk and Insurance Services Segment [Member]
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	10	$ 12
Consulting Segment [Member]
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	$ 2	$ 3