Upper limit for tender purchase of aggregate principal amount.

Name:	mmc_UpperLimitForTenderPurchaseOfAggregatePrincipalAmount
Namespace Prefix:	mmc
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.13,16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Interest rate stated in the contractual debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Sum of the carrying values as of the balance sheet date of all long-term debt, which is debt initially having maturities due after one year from the balance sheet date or beyond the operating cycle, if longer, but excluding the portions thereof scheduled to be repaid within one year or the normal operating cycle, if longer plus capital lease obligations due to be paid more than one year after the balance sheet date.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section H

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebtAndCapitalLeaseObligations
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Total of the portions of the carrying amounts as of the balance sheet date of long-term debt, which may include notes payable, bonds payable, debentures, mortgage loans, and commercial paper, which are scheduled to be repaid within one year or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20

-Article 5

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19,20)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	credit
Period Type:	instant

Debt (Schedule Of Outstanding Debt) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011	Jul. 15, 2011	Dec. 31, 2010	Sep. 30, 2011 Senior Debt Obligations Due 2012 [Member]	Dec. 31, 2010 Senior Debt Obligations Due 2012 [Member]	Sep. 30, 2011 Senior Debt Obligations Due 2013 [Member]	Dec. 31, 2010 Senior Debt Obligations Due 2013 [Member]	Sep. 30, 2011 Senior Debt Obligations Due 2033 [Member]	Dec. 31, 2010 Senior Debt Obligations Due 2033 [Member]	Sep. 30, 2011 Senior Debt Obligations Due 2014 [Member]	Jun. 27, 2011 Senior Debt Obligations Due 2014 [Member]	Dec. 31, 2010 Senior Debt Obligations Due 2014 [Member]	Sep. 30, 2011 Senior Debt Obligations Due 2015 [Member]	Jun. 27, 2011 Senior Debt Obligations Due 2015 [Member]	Dec. 31, 2010 Senior Debt Obligations Due 2015 [Member]	Sep. 30, 2011 Senior Debt Obligations Due 2019 [Member]	Dec. 31, 2010 Senior Debt Obligations Due 2019 [Member]	Sep. 30, 2011 Senior Debt Obligations Due 2021 [Member]	Sep. 30, 2011 Mortgage Due 2035 [Member]	Dec. 31, 2010 Mortgage Due 2035 [Member]	Sep. 30, 2011 Other [Member]	Dec. 31, 2010 Other [Member]
Long-term debt, current and noncurrent	$ 2,930		$ 3,034
Current portion of long-term debt	260	600	8
Long-term debt and capital lease obligations	2,670		3,026	251	253	250	250	296	296	326	650	648	479	750	747	398	398	496	433	439	1	3
Debt instrument, interest rate, stated percentage				6.25%		4.85%		5.875%		5.375%	5.375%		5.75%	5.75%		9.25%		4.80%	5.70%
Debt instrument, interest rate, effective percentage				5.10%
Upper limit for tender purchase of aggregate principal amount											$ 500