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Pension and Other Retirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Dividends paid on ESOP		$ 1	$ 1	$ 1
Moody's share held in ESOP		411,100	435,500
U.S. Plans
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%
Defined contribution compensation expense		$ 43	$ 27	$ 43
U.S. Plans \| Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Expected rate of return		5.65%	4.50%	5.40%
Employer contributions		$ 8	$ 20
U.S. Plans \| Other Retirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Expected rate of return		0.00%	0.00%	0.00%
Employer contributions		$ 0	$ 0
U.S. Plans \| Equity Securities \| Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage		49.00%
U.S. Plans \| Equity Securities \| Pension Plans \| Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage		44.00%
U.S. Plans \| Equity Securities \| Pension Plans \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage		54.00%
U.S. Plans \| Fixed Income Securities \| Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage		45.00%
U.S. Plans \| Fixed Income Securities \| Pension Plans \| Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage		40.00%
U.S. Plans \| Fixed Income Securities \| Pension Plans \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage		50.00%
U.S. Plans \| Other Investments \| Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage		6.00%
U.S. Plans \| Other Investments \| Pension Plans \| Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage		3.00%
U.S. Plans \| Other Investments \| Pension Plans \| Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Plan asset, target asset allocation percentage		9.00%
Non-U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Defined contribution compensation expense		$ 25	26	$ 24
Funded Plan \| U.S. Plans \| Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Employer contributions			16
Anticipated contribution to plans		99
Unfunded Plan \| U.S. Plans \| Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Employer contributions		8	$ 5
Anticipated contribution to plans		$ 6
Forecast \| U.S. Plans \| Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Abstract]
Expected rate of return	4.45%