Net uncollected premiums and claims and the related payables are not assets and liabilities of MMC and are not included in the accompanying consolidated balance sheets.

No definition available.

Name:	mmc_UncollectedPremiumsandClaimsNetofRelatedPayables
Namespace Prefix:	mmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fiduciary Assets And Liabilities (Details) (USD $)			9 Months Ended		3 Months Ended		9 Months Ended
Fiduciary Assets And Liabilities (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Risk and Insurance Services Segment [Member]	Sep. 30, 2012 Risk and Insurance Services Segment [Member]	Sep. 30, 2013 Consulting Segment [Member]	Sep. 30, 2012 Consulting Segment [Member]	Sep. 30, 2013 Consulting Segment [Member]	Sep. 30, 2012 Consulting Segment [Member]	Sep. 30, 2013 Mercer Consulting Group [Member]
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds			$ 21,000,000	$ 31,000,000	$ 1,000,000	$ 1,000,000	$ 3,000,000	$ 2,000,000
Net uncollected premiums and claims receivable and payable	9,000,000,000	9,100,000,000
Assets in trusts or funds for management or trustee fee									$ 17,000,000,000