CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Millions, $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Capital Surplus	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock	Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss Cumulative Effect, Period of Adoption, Adjustment	Total Moody’s Shareholders’ Equity	Total Moody’s Shareholders’ Equity Cumulative Effect, Period of Adoption, Adjustment	Non- Controlling Interests
Beginning Balance (in shares) at Dec. 31, 2017			342.9				151.9
Beginning Balance at Dec. 31, 2017	$ (115)		$ 3	$ 529	$ 7,465		$ (8,153)	$ (172)		$ (328)		$ 213
Beginning Balance (Accounting Standards Update 2014-09) at Dec. 31, 2017		$ 156				$ 156					$ 156
Beginning Balance (Accounting Standards Update 2016-01) at Dec. 31, 2017		0				2			$ (2)		0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,320				1,310					1,310		10
Dividends	(343)				(339)					(339)		(4)
Stock-based compensation	131			131						131
Shares issued for stock-based compensation plans at average cost, net (in shares)							1.5
Shares issued for stock-based compensation plans at average cost, net	(16)			(59)			$ 43			(16)
Treasury shares repurchased (in shares)							(1.2)
Treasury shares repurchased	(203)						$ (203)			(203)
Currency translation adjustment, net of net investment hedge activity	(281)							(259)		(259)		(22)
Net actuarial gains and prior service cost	4							4		4
Amortization of prior service costs and actuarial losses	4							4		4
Net realized and unrealized gain on cash flow hedges	(1)							(1)		(1)
Ending Balance (in shares) at Dec. 31, 2018			342.9				151.6
Ending Balance at Dec. 31, 2018	$ 656	$ 0	$ 3	601	8,594	20	$ (8,313)	(426)	$ (20)	459	0	197
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201802Member	us-gaap:AccountingStandardsUpdate201802Member
Net income	$ 1,429				1,422					1,422		7
Dividends	(383)				(380)					(380)		(3)
Stock-based compensation	136			136						136
Shares issued for stock-based compensation plans at average cost, net (in shares)							1.6
Shares issued for stock-based compensation plans at average cost, net	(32)			(70)			$ 38			(32)
Purchase of noncontrolling interest	(12)			(9)						(9)		(3)
Non-controlling interest resulting from majority acquisition of Vigeo Eiris	17									0		17
Treasury shares repurchased (in shares)							(5.2)
Treasury shares repurchased	(991)			(16)			$ (975)			(991)
Currency translation adjustment, net of net investment hedge activity	33							29		29		4
Net actuarial gains and prior service cost	(24)							(24)		(24)
Amortization of prior service costs and actuarial losses	2							2		2
Ending Balance (in shares) at Dec. 31, 2019			342.9				155.2
Ending Balance at Dec. 31, 2019	$ 831	$ (2)	$ 3	642	9,656	$ (2)	$ (9,250)	(439)		612	$ (2)	219
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Net income	$ 1,777
Dividends	(424)				(421)					(421)		(3)
Stock-based compensation	154			154						154
Shares issued for stock-based compensation plans at average cost, net (in shares)							1.4
Shares issued for stock-based compensation plans at average cost, net	(53)			(58)			$ 5			(53)
Purchase of noncontrolling interest	(17)			(3)						(3)		(14)
Treasury shares repurchased (in shares)							(2.0)
Treasury shares repurchased	(503)			0			$ (503)			(503)
Currency translation adjustment, net of net investment hedge activity	74							82		82		(8)
Net actuarial gains and prior service cost	(32)							(32)		(32)
Amortization of prior service costs and actuarial losses	6							6		6
Net realized and unrealized gain on cash flow hedges	(49)							(49)		(49)
Ending Balance (in shares) at Dec. 31, 2020			342.9				155.8
Ending Balance at Dec. 31, 2020	1,763		$ 3	$ 735	11,011		$ (9,748)	$ (432)		1,569		194
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss), Including Portion Attributable to Nonredeemable Noncontrolling Interest	$ 1,778				$ 1,778					$ 1,778		$ 0