Long-term Commitments (Future Minimum Rental Commitments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Long-Term Commitments [Abstract]
2012	$ 101
2013	59
2014	47
Subsequent years	142
Total	$ 349

Fixed future minimum commitments due within one year of the balance sheet date

No definition available.

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