Claims, Lawsuits And Other Contingencies (Details) (GBP £) In Millions	Jun. 30, 2011
Claims, Lawsuits And Other Contingencies
Reinsured policies covered by related party guarantee	£ 40

Name:

mmc_ReinsuredPoliciesCoveredByRelatedPartyGuarantee

Namespace Prefix:

mmc

Data Type:

xbrli:monetaryItemType

Balance Type:

credit

Period Type:

instant