CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) shares in Millions, $ in Millions	Total	Total Moody's Shareholders' Equity	Common Stock	Capital Surplus	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Loss	Non- Controlling Interests	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjustment Total Moody's Shareholders' Equity	Cumulative Effect, Period of Adoption, Adjustment Retained Earnings
Beginning Balance (in shares) at Dec. 31, 2019			342.9			155.2
Beginning Balance at Dec. 31, 2019	$ 831	$ 612	$ 3	$ 642	$ 9,656	$ (9,250)	$ (439)	$ 219	$ (2)	$ (2)	$ (2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,464	1,464			1,464
Net Income	1,463
Dividends	(315)	(314)			(314)			(1)
Stock-based compensation	110	110		110
Shares issued for stock-based compensation plans at average cost, net (in shares)						1.2
Shares issued for stock-based compensation plans at average cost, net	(53)	(53)		(51)		$ (2)
Purchase of noncontrolling interest	(17)	(2)		(2)				(15)
Treasury shares repurchased (in shares)						(1.1)
Treasury shares repurchased	(253)	(253)				$ (253)
Currency translation adjustment, net of net investment hedge activity (net of tax)	(31)	(19)					(19)	(12)
Net actuarial gains and prior service cost (net of tax)	(1)	(1)					(1)
Amortization of prior service costs/actuarial losses and settlement charge	4	4					4
Net realized and unrealized gain (loss) on cash flow hedges (net of tax)	(49)	(49)					(49)
Ending Balance (in shares) at Sep. 30, 2020			342.9			155.1
Ending Balance at Sep. 30, 2020	1,688	1,497	$ 3	699	10,804	$ (9,505)	(504)	191
Beginning Balance (in shares) at Jun. 30, 2020			342.9			155.2
Beginning Balance at Jun. 30, 2020	1,232	1,041	$ 3	651	10,442	$ (9,513)	(542)	191
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	467	467			467
Dividends	(106)	(105)			(105)			(1)
Stock-based compensation	38	38		38
Shares issued for stock-based compensation plans at average cost, net (in shares)						0.1
Shares issued for stock-based compensation plans at average cost, net	18	18		10		$ 8
Currency translation adjustment, net of net investment hedge activity (net of tax)	44	43					43	1
Net actuarial gains and prior service cost (net of tax)	(7)	(7)					(7)
Amortization of prior service costs/actuarial losses and settlement charge	2	2					2
Ending Balance (in shares) at Sep. 30, 2020			342.9			155.1
Ending Balance at Sep. 30, 2020	1,688	1,497	$ 3	699	10,804	$ (9,505)	(504)	191
Beginning Balance (in shares) at Dec. 31, 2020			342.9			155.8
Beginning Balance at Dec. 31, 2020	1,763	1,569	$ 3	735	11,011	$ (9,748)	(432)	194
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	1,787	1,787			1,787
Dividends	(350)	(347)			(347)			(3)
Stock-based compensation	127	127		127
Shares issued for stock-based compensation plans at average cost, net (in shares)						0.7
Shares issued for stock-based compensation plans at average cost, net	(48)	(48)		(30)		$ (18)
Treasury shares repurchased (in shares)						(1.9)
Treasury shares repurchased	(628)	(628)				$ (628)
Currency translation adjustment, net of net investment hedge activity (net of tax)	(49)	(47)					(47)	(2)
Net actuarial gains and prior service cost (net of tax)	3	3					3
Amortization of prior service costs/actuarial losses and settlement charge	12	12					12
Net realized and unrealized gain (loss) on cash flow hedges (net of tax)	2	2					2
Ending Balance (in shares) at Sep. 30, 2021			342.9			157.0
Ending Balance at Sep. 30, 2021	2,619	2,430	$ 3	832	12,451	$ (10,394)	(462)	189
Beginning Balance (in shares) at Jun. 30, 2021			342.9			156.7
Beginning Balance at Jun. 30, 2021	2,380	2,186	$ 3	784	12,094	$ (10,270)	(425)	194
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	474	474			474
Dividends	(119)	(117)			(117)			(2)
Stock-based compensation	41	41		41
Shares issued for stock-based compensation plans at average cost, net (in shares)						0.0
Shares issued for stock-based compensation plans at average cost, net	8	8		7		$ 1
Treasury shares repurchased (in shares)						(0.3)
Treasury shares repurchased	(125)	(125)				$ (125)
Currency translation adjustment, net of net investment hedge activity (net of tax)	(46)	(43)					(43)	(3)
Net actuarial gains and prior service cost (net of tax)	3	3					3
Amortization of prior service costs/actuarial losses and settlement charge	2	2					2
Net realized and unrealized gain (loss) on cash flow hedges (net of tax)	1	1					1
Ending Balance (in shares) at Sep. 30, 2021			342.9			157.0
Ending Balance at Sep. 30, 2021	$ 2,619	$ 2,430	$ 3	$ 832	$ 12,451	$ (10,394)	$ (462)	$ 189