Long-term Commitments (Operating Lease Agreements) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Gross Rental Commitments
2015, Net Rental Commitments	$ 365
2016, Gross Rental Commitments	334
2017, Gross Rental Commitments	293
2018, Gross Rental Commitments	258
2019, Gross Rental Commitments	212
Subsequent years, Gross Rental Commitments	1,010
Rental from Subleases
2015, Rentals from Subleases	48
2016, Rentals from Subleases	48
2017, Rentals from Subleases	45
2018, Rentals from Subleases	42
2019, Rentals from Subleases	35
Subsequent years, Rentals from Subleases	36
Net Rental Commitments
2015, Net Rental Commitments	317
2016, Net Rental Commitments	286
2017, Net Rental Commitments	248
2018, Net Rental Commitments	216
2019, Net Rental Commitments	177
Subsequent years, Net Rental Commitments	$ 974

Name:	mmc_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsAbstract
Namespace Prefix:	mmc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Operating Leases Future Minimum Payments Due, Future Minimum Sublease Rentals Thereafter

+ References

No definition available.

+ Details

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

- Definition

Operating Leases Future Minimum Payments Due, Future Minimum Sublease Rentals Within Five Years

+ References

No definition available.

+ Details

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

- Definition

Operating Leases Future Minimum Payments Due, Future Minimum Sublease Rentals Within Four Years

+ References

No definition available.

+ Details

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

- Definition

Operating Leases Future Minimum Payments Due, Future Minimum Sublease Rentals Within One Year

+ References

No definition available.

+ Details

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

- Definition

Operating Leases Future Minimum Payments Due, Future Minimum Sublease Rentals Within Three Years

+ References

No definition available.

+ Details

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

- Definition

Operating Leases Future Minimum Payments Due, Future Minimum Sublease Rentals Within Two Years

+ References

No definition available.

+ Details

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

- Details

Name:	mmc_OperatingLeasesFutureMinimumPaymentsDueNetofSubleaseReceivableAbstract
Namespace Prefix:	mmc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Operating Leases, Future Minimum Payments, Net of Sublease Receivable, Due Current

+ References

No definition available.

+ Details

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

- Definition

Operating Leases, Future Minimum Payments, Net of Sublease Receivable, Due In Five Years

+ References

No definition available.

+ Details

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

- Definition

Operating Leases, Future Minimum Payments, Net of Sublease Receivable, Due In Four Years

+ References

No definition available.

+ Details

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

- Definition

Operating Leases Future Minimum Payments, Net of Sublease Receivable, Due In Three Years

+ References

No definition available.

+ Details

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

- Definition

Operating Leases, Future Minimum Payments, Net of Sublease Receivable, Due In Two Years

+ References

No definition available.

+ Details

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

- Definition

Operating Leases, Future Minimum Payments, Net of Sublease Receivable, Due Thereafter

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 10

-Section 55

-Paragraph 40

-Subparagraph (Note 3)

-URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant