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 $)			3 Months Ended
Fiduciary Assets and Liabilities (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Risk and Insurance Services Segment [Member]	Mar. 31, 2013 Risk and Insurance Services Segment [Member]	Mar. 31, 2014 Consulting Segment [Member]	Mar. 31, 2013 Consulting Segment [Member]	Mar. 31, 2014 Mercer Consulting Group [Member]
Fiduciary Assets and Liabilities [Line Items]
Interest on fiduciary funds			$ 6,000,000	$ 8,000,000	$ 1,000,000	$ 1,000,000
Net uncollected premiums and claims receivable and payable	8,700,000,000	8,200,000,000
Assets in trusts or funds for management or trustee fee							$ 17,000,000,000