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	Cumulative Effect, Period of Adoption, Adjustment Accumulated Other Comprehensive Loss
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/ (loss)	997	997			997
Dividends	(209)	(209)			(209)
Stock-based compensation	72	72		72
Shares issued for stock-based compensation plans at average cost, net (in shares)						1.1
Shares issued for stock-based compensation plans at average cost, net	(71)	(71)		(61)		$ (10)
Purchase of noncontrolling interest	(17)	(2)		(2)				(15)
Treasury shares repurchased (in shares)						(1.1)
Treasury shares repurchased	(253)	(253)		0		$ (253)
Currency translation adjustment, net of net investment hedge activity (net of tax)	(75)	(62)					(62)	(13)
Net actuarial gains and prior service cost (net of tax)	6	6					6
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)	(49)	(49)					(49)
Ending Balance (in shares) at Jun. 30, 2020			342.9			155.2
Ending Balance at Jun. 30, 2020	1,232	1,041	$ 3	651	10,442	$ (9,513)	(542)	191
Beginning Balance (in shares) at Mar. 31, 2020			342.9			155.4
Beginning Balance at Mar. 31, 2020	799	582	$ 3	616	10,041	$ (9,524)	(554)	217
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	510	509			509			1
Net income/ (loss)	509
Dividends	(108)	(108)			(108)			0
Stock-based compensation	35	35		35
Shares issued for stock-based compensation plans at average cost, net (in shares)						0.2
Shares issued for stock-based compensation plans at average cost, net	13	13		2		$ 11
Purchase of noncontrolling interest	(17)	(2)		(2)				(15)
Currency translation adjustment, net of net investment hedge activity (net of tax)	6	18					18	(12)
Net actuarial gains and prior service cost (net of tax)	7	7					7
Amortization of prior service costs/actuarial losses and settlement charge	1	1					1
Net realized and unrealized gain (loss) on cash flow hedges (net of tax)	(14)	(14)					(14)
Ending Balance (in shares) at Jun. 30, 2020			342.9			155.2
Ending Balance at Jun. 30, 2020	1,232	1,041	$ 3	651	10,442	$ (9,513)	(542)	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/ (loss)	1,313	1,313			1,313
Dividends	(231)	(230)			(230)			(1)
Stock-based compensation	86	86		86
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	(56)	(56)		(37)		$ (19)
Treasury shares repurchased (in shares)						(1.6)
Treasury shares repurchased	(503)	(503)				$ (503)
Currency translation adjustment, net of net investment hedge activity (net of tax)	(3)	(4)					(4)	1
Net actuarial gains and prior service cost (net of tax)	0
Amortization of prior service costs/actuarial losses and settlement charge	10	10					10
Net realized and unrealized gain (loss) on cash flow hedges (net of tax)	1	1					1
Ending Balance (in shares) at Jun. 30, 2021			342.9			156.7
Ending Balance at Jun. 30, 2021	2,380	2,186	$ 3	784	12,094	$ (10,270)	(425)	194
Beginning Balance (in shares) at Mar. 31, 2021			342.9			155.7
Beginning Balance at Mar. 31, 2021	2,225	2,030	$ 3	739	11,632	$ (9,904)	(440)	195
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income/ (loss)	577	577			577
Dividends	(115)	(115)			(115)
Stock-based compensation	41	41		41
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	9	9		4		$ 5
Treasury shares repurchased (in shares)						(1.1)
Treasury shares repurchased	(371)	(371)				$ (371)
Currency translation adjustment, net of net investment hedge activity (net of tax)	6	7					7	(1)
Net actuarial gains and prior service cost (net of tax)	0
Amortization of prior service costs/actuarial losses and settlement charge	8	8					8
Ending Balance (in shares) at Jun. 30, 2021			342.9			156.7
Ending Balance at Jun. 30, 2021	$ 2,380	$ 2,186	$ 3	$ 784	$ 12,094	$ (10,270)	$ (425)	$ 194