Comprehensive Income And Accumulated Other Comprehensive Loss - Reclassification out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating income, net	$ 10	$ 10	$ 38	$ 12
Interest expense, net	(53)	(46)	(153)	(149)
Operating expense	364	350	1,066	1,032
Selling, general and administrative	271	292	879	848
Total before income taxes	599	513	1,829	1,357
Provision for income taxes	(132)	(130)	(366)	(290)
Net income attributable to Moody's	467	380	1,464	1,062
Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Net income attributable to Moody's	(2)	0	(5)	(1)
Losses on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Total before income taxes		0		0
Provision for income taxes	0	0	0	0
Net income attributable to Moody's	0	0	(1)	0
Losses on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income | Cross-currency swap
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating income, net		0		0
Losses on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income | Interest rate contract
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating income, net	0		(1)
Interest expense, net		0		0
Net Investment Hedges | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Provision for income taxes		0		0
Net income attributable to Moody's		1		1
Net Investment Hedges | Reclassification out of Accumulated Other Comprehensive Income | FX forwards
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating income, net		1		1
Total pension and other retirement benefits | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Total before income taxes	(2)	(1)	(5)	(3)
Provision for income taxes	0	0	1	1
Net income attributable to Moody's	(2)	(1)	(4)	(2)
Amortization of actuarial losses and prior service costs included in net income | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Operating expense	(1)	(1)	(3)	(2)
Selling, general and administrative	$ (1)	$ 0	$ (2)	$ (1)