Comprehensive Income And Accumulated Other Comprehensive Income - Reclassification out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating income, net	$ 16	$ 0	$ 28	$ 2
Operating expense	362	340	702	682
Selling, general and administrative	307	275	608	556
Total before income taxes	666	432	1,230	844
Provision for income taxes	(157)	(121)	(234)	(159)
Net income attributable to Moody's	509	310	997	683
Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Net income attributable to Moody's	(1)	(1)	(3)	(1)
Losses on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Provision for income taxes	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)
Total pension and other retirement benefits | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Total before income taxes	(1)	(1)	(3)	(2)
Provision for income taxes	0	0	1	1
Net income attributable to Moody's	(1)	(1)	(2)	(1)
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)	0	(2)	(1)
Selling, general and administrative	0	$ (1)	(1)	$ (1)
Gains (losses) on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Net income attributable to Moody's	$ 0		$ (1)