CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Capital Surplus	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Loss	Total Moody's Shareholders' Equity	Non- Controlling Interests	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjustment Retained Earnings	Cumulative Effect, Period of Adoption, Adjustment Accumulated Other Comprehensive Loss	Cumulative Effect, Period of Adoption, Adjustment Total Moody's Shareholders' Equity
Beginning Balance (in shares) at Dec. 31, 2018		342.9			151.6
Beginning Balance at Dec. 31, 2018	$ 656	$ 3	$ 601	$ 8,594	$ (8,313)	$ (426)	$ 459	$ 197	$ 0	$ 20	$ (20)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,067			1,062			1,062	5
Dividends	(286)			(285)			(285)	(1)
Stock-based compensation	103		103				103
Shares issued for stock-based compensation plans at average cost, net	(39)		(71)		$ 32		(39)
Shares issued for stock-based compensation plans at average cost, net (in shares)					1.5
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)					(4.1)
Treasury shares repurchased	(728)		(16)		$ (712)		(728)
Currency translation adjustment, net of net investment hedge activity (net of tax)	(82)					(92)	(92)	10
Net actuarial gains (losses) and prior service costs	(1)					(1)	(1)
Amortization of prior service costs and actuarial losses	2					2	2
Ending Balance (in shares) at Sep. 30, 2019		342.9			154.2
Ending Balance at Sep. 30, 2019	$ 697	$ 3	608	9,391	$ (8,993)	(537)	472	225
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Beginning Balance (in shares) at Dec. 31, 2018		342.9			151.6
Beginning Balance at Dec. 31, 2018	$ 656	$ 3	601	8,594	$ (8,313)	(426)	459	197	0	20	$ (20)	0
Ending Balance (in shares) at Dec. 31, 2019		342.9			155.2
Ending Balance at Dec. 31, 2019	831	$ 3	642	9,656	$ (9,250)	(439)	612	219	(2)	(2)		(2)
Beginning Balance (in shares) at Jun. 30, 2019		342.9			153.7
Beginning Balance at Jun. 30, 2019	575	$ 3	575	9,108	$ (8,887)	(446)	353	222
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	383			380			380	3
Dividends	(97)			(97)			(97)	0
Stock-based compensation	33		33				33
Shares issued for stock-based compensation plans at average cost, net	7		0		$ 7		7
Shares issued for stock-based compensation plans at average cost, net (in shares)					0.2
Treasury shares repurchased (in shares)					(0.7)
Treasury shares repurchased	(113)		0		$ (113)		(113)
Currency translation adjustment, net of net investment hedge activity (net of tax)	(92)					(92)	(92)	0
Net actuarial gains (losses) and prior service costs	0
Amortization of prior service costs and actuarial losses	1					1	1
Ending Balance (in shares) at Sep. 30, 2019		342.9			154.2
Ending Balance at Sep. 30, 2019	697	$ 3	608	9,391	$ (8,993)	(537)	472	225
Beginning Balance (in shares) at Dec. 31, 2019		342.9			155.2
Beginning Balance at Dec. 31, 2019	831	$ 3	642	9,656	$ (9,250)	(439)	612	219	$ (2)	$ (2)		$ (2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,464			1,464			1,464	0
Dividends	(315)			(314)			(314)	(1)
Stock-based compensation	110		110				110
Shares issued for stock-based compensation plans at average cost, net	(53)		(51)		$ (2)		(53)
Shares issued for stock-based compensation plans at average cost, net (in shares)					1.2
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 (losses) and prior service costs	(1)					(1)	(1)
Amortization of prior service costs and actuarial losses	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	$ 3	699	10,804	$ (9,505)	(504)	1,497	191
Beginning Balance (in shares) at Jun. 30, 2020		342.9			155.2
Beginning Balance at Jun. 30, 2020	1,232	$ 3	651	10,442	$ (9,513)	(542)	1,041	191
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	467			467			467	0
Dividends	(106)			(105)			(105)	(1)
Stock-based compensation	38		38				38
Shares issued for stock-based compensation plans at average cost, net	18		10		$ 8		18
Shares issued for stock-based compensation plans at average cost, net (in shares)					0.1
Currency translation adjustment, net of net investment hedge activity (net of tax)	44					43	43	1
Net actuarial gains (losses) and prior service costs	(7)					(7)	(7)
Amortization of prior service costs and actuarial losses	2					2	2
Ending Balance (in shares) at Sep. 30, 2020		342.9			155.1
Ending Balance at Sep. 30, 2020	$ 1,688	$ 3	$ 699	$ 10,804	$ (9,505)	$ (504)	$ 1,497	$ 191