Summary of Changes in Benefit Obligations and Fair Value of Plan Assets for Post-Retirement Plans (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in fair value of plan assets:
Fair value of plan assets, beginning balance	$ 248.5
Fair value of plan assets, ending balance	260.9	$ 248.5
Amounts recognized on the consolidated balance sheets:
Pension and retirement benefits liability-current	(5.4)	(5.1)
Pension and retirement benefits liability-non current	(228.3)	(244.8)
Pension Plans [Member]
Change in benefit obligation:
Benefit obligation, beginning of the period	(461.8)	(347.1)
Service cost	(21.6)	(18.4)	$ (19.8)
Interest cost	(16.9)	(16.5)	(13.5)
Plan amendments	6.4
Benefits paid	7.5	6.4
Actuarial gain (loss)	1.9	(8.3)
Assumption changes	25.3	(77.9)
Benefit obligation, end of the period	(459.2)	(461.8)	(347.1)
Change in fair value of plan assets:
Fair value of plan assets, beginning balance	248.4	204.6
Actual return on plan assets	(4.5)	12.9
Benefits paid	(7.4)	(6.4)
Employer contributions	24.4	37.3
Fair value of plan assets, ending balance	260.9	248.4	204.6
Funded status of the plans	(198.3)	(213.4)
Amounts recognized on the consolidated balance sheets:
Pension and retirement benefits liability-current	(4.4)	(4.3)
Pension and retirement benefits liability-non current	(193.9)	(209.1)
Net amount recognized	(198.3)	(213.4)
Accumulated benefit obligation, end of the period	(405.3)	(396.3)
Other Retirement Plans [Member]
Change in benefit obligation:
Benefit obligation, beginning of the period	(26.7)	(20.7)
Service cost	(2.2)	(1.7)	(1.7)
Interest cost	(1.0)	(0.9)	(0.8)
Plan participants' contributions	(0.4)	(0.4)
Plan amendments	1.2
Benefits paid	0.8	0.6
Actuarial gain (loss)	(0.1)	(0.1)
Assumption changes	1.4	(3.5)
Benefit obligation, end of the period	(27.0)	(26.7)	$ (20.7)
Change in fair value of plan assets:
Benefits paid	(0.8)	(0.6)
Employer contributions	0.4	0.2
Plan participants' contributions	0.4	0.4
Funded status of the plans	(27.0)	(26.7)
Amounts recognized on the consolidated balance sheets:
Pension and retirement benefits liability-current	(1.0)	(0.8)
Pension and retirement benefits liability-non current	(26.0)	(25.9)
Net amount recognized	$ (27.0)	$ (26.7)

No definition available.

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Namespace Prefix:	mco_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Accumulated Benefit Obligation
-URI http://asc.fasb.org/extlink&oid=6503844

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920