Fiduciary Assets and Liabilities (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fiduciary Assets and Liabilities [Line Items]
Net uncollected premiums and claims receivable and payable	$ 8,100,000,000		$ 7,300,000,000
Mercer Consulting Group [Member]
Fiduciary Assets and Liabilities [Line Items]
Assets in trusts or funds for management or trustee fee	125,000,000,000
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	26,000,000,000
Risk and Insurance Services Segment [Member]
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	5,000,000	6,000,000
Consulting Segment [Member]
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds	$ 1,000,000	$ 1,000,000