Claims, Lawsuits And Other Contingencies Claims, Lawsuits And Other Contingencies (Details) (Other Contingencies-Guarantees [Member], GBP £) In Millions, unless otherwise specified	Dec. 31, 2014
Other Contingencies-Guarantees [Member]
Loss Contingencies [Line Items]
Amount reinsured by third party	£ 40

Reinsurance Policy, Amount Reinsured By Third Party

No definition available.

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