Claims, Lawsuits And Other Contingencies (Details) (GBP £) In Millions, unless otherwise specified	Dec. 31, 2011
Commitments and Contingencies Disclosure [Abstract]
Reinsured policies covered by related party guarantee	£ 40

Reinsured Policies Covered By Related Party Guarantee

No definition available.

Name:	mmc_ReinsuredPoliciesCoveredByRelatedPartyGuarantee
Namespace Prefix:	mmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant