Discount allowed to employees on purchase of shares under ESPP plan.

No definition available.

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- Definition

Lowest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	mco_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Increase in number of shares reserved for issuance as approved by shareholders.

+ References

No definition available.

+ Details

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Namespace Prefix:	mco_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Stock options exercisable period maximum.

+ References

No definition available.

+ Details

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Namespace Prefix:	mco_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate number of common shares reserved for future issuance.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.29)

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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph h

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, using a decimal to express in number of years.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the period of time over which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, which may be expressed in a variety of ways (for example, in years, month and year).

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum vesting term over which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The minimum vesting term over which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	duration

- Details

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Balance Type:	na
Period Type:	duration

- Definition

The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 65

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Stock-Based Compensation Plans (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 years	Dec. 31, 2011 Employee Stock Purchase Plan [Member] years	Dec. 31, 2010 Employee Stock Purchase Plan [Member]	Dec. 31, 2009 Employee Stock Purchase Plan [Member]	Dec. 31, 2011 Directors [Member]	Dec. 31, 2011 Options [Member]	Dec. 31, 2011 Restricted Stock [Member]	Dec. 31, 2011 Instruments Other Than Options [Member]	Dec. 31, 2011 Performance-Based Restricted Stock [Member]	Dec. 31, 2011 1998 Plan [Member] Options [Member]	Apr. 20, 2010 2001 Plan [Member]	Dec. 31, 2011 2001 Plan [Member] Options [Member]	Dec. 31, 2011 2001 Plan [Member] Instruments Other Than Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance		6.0			0.8
Maximum number of shares for grants of awards										33.0		35.6	15.0
Increase in common stock reserved for issuance											7.0
Stock options exercisable period, maximum, years	10				10
Stock options vesting period, years	4				3				3
Stock options vesting period, minimum, years	1				1
Stock options vesting period, maximum, years	4				1
Unrecognized compensation expense						$ 19.6	$ 43.3	$ 13.0
Weighted average period to recognize expense						1.4	1.7	0.9
Discount allowed to employees on purchase of shares under ESPP plan		5.00%	5.00%	5.00%
ESPP plans elected by plan participants, minimum		1.00%
ESPP plans elected by plan participants, maximum		10.00%