Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year.

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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Amount of cost for defined contribution plan.

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Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

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Dividends paid on stock held by an employee stock ownership plan.

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Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan.

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Pension and Other Retirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Expected rate of return	5.40%
Moody's share held in ESOP	457,000	471,000
U.S. Defined Contribution Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Defined contribution compensation expense	$ 43.3	$ 28.3	$ 21.1
International Defined Contribution Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Defined contribution compensation expense	$ 23.9	24.5	26.7
Defined Contribution Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Employee contribution percentage of employee contribution in participation	50.00%
Maximum employee contribution in profit participation plan	3.00%
Dividends paid on ESOP	$ 0.7	0.7	$ 0.7
Funded Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Employer contributions	25.9	22.4
Unfunded Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Employer contributions	5.1	4.0
Anticipated contribution to plans	4.9
Other Retirement Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Employer contributions	1.0	$ 0.5
Anticipated contribution to plans	$ 1.0
2018 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Expected rate of return	4.50%
Equity Securities [Member] \| Revised Investment Policy [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage	53.00%
Equity Securities [Member] \| Revised Investment Policy [Member] \| Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage	58.00%
Equity Securities [Member] \| Revised Investment Policy [Member] \| Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage	48.00%
Fixed Income Securities [Member] \| Revised Investment Policy [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage	40.00%
Fixed Income Securities [Member] \| Revised Investment Policy [Member] \| Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage	45.00%
Fixed Income Securities [Member] \| Revised Investment Policy [Member] \| Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage	35.00%
Other Investments [Member] \| Revised Investment Policy [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage	7.00%
Other Investments [Member] \| Revised Investment Policy [Member] \| Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage	10.00%
Other Investments [Member] \| Revised Investment Policy [Member] \| Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage	4.00%