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	685			683			683	2
Dividends	(190)			(189)			(189)	(1)
Stock-based compensation	70		70				70
Shares issued for stock-based compensation plans at average cost, net	(46)		(71)		$ 25		(46)
Shares issued for stock-based compensation plans at average cost, net (in shares)					1.3
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)					(3.4)
Treasury shares repurchased	(615)		(16)		$ (599)		(615)
Currency translation adjustment, net of net investment hedge activity (net of tax)	10					0	0	10
Net actuarial gains (losses) and prior service costs	(1)					(1)	(1)
Amortization of prior service costs and actuarial losses	1					1	1
Ending Balance (in shares) at Jun. 30, 2019		342.9			153.7
Ending Balance at Jun. 30, 2019	$ 575	$ 3	575	9,108	$ (8,887)	(446)	353	222
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 Mar. 31, 2019		342.9			153.3
Beginning Balance at Mar. 31, 2019	325	$ 3	436	8,893	$ (8,754)	(455)	123	202
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	311			310			310	1
Dividends	(96)			(95)			(95)	(1)
Stock-based compensation	34		34				34
Shares issued for stock-based compensation plans at average cost, net	14		(4)		$ 18		14
Shares issued for stock-based compensation plans at average cost, net (in shares)					0.3
Non-controlling interest resulting from majority acquisition of Vigeo Eiris	17						0	17
Treasury shares repurchased (in shares)					(0.7)
Treasury shares repurchased	(42)		109		$ (151)		(42)
Currency translation adjustment, net of net investment hedge activity (net of tax)	13					10	10	3
Net actuarial gains (losses) and prior service costs	(2)					(2)	(2)
Amortization of prior service costs and actuarial losses	1					1	1
Ending Balance (in shares) at Jun. 30, 2019		342.9			153.7
Ending Balance at Jun. 30, 2019	575	$ 3	575	9,108	$ (8,887)	(446)	353	222
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	997			997			997
Dividends	(209)			(209)			(209)
Stock-based compensation	72		72				72
Shares issued for stock-based compensation plans at average cost, net	(71)		(61)		$ (10)		(71)
Shares issued for stock-based compensation plans at average cost, net (in shares)					1.1
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)	(75)					(62)	(62)	(13)
Net actuarial gains (losses) and prior service costs	6					6	6
Amortization of prior service costs and actuarial losses	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	$ 3	651	10,442	$ (9,513)	(542)	1,041	191
Beginning Balance (in shares) at Mar. 31, 2020		342.9			155.4
Beginning Balance at Mar. 31, 2020	799	$ 3	616	10,041	$ (9,524)	(554)	582	217
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	510			509			509	1
Dividends	(108)			(108)			(108)
Stock-based compensation	35		35				35
Shares issued for stock-based compensation plans at average cost, net	13		2		$ 11		13
Shares issued for stock-based compensation plans at average cost, net (in shares)					0.2
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 (losses) and prior service costs	7					7	7
Amortization of prior service costs and actuarial losses	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	$ 3	$ 651	$ 10,442	$ (9,513)	$ (542)	$ 1,041	$ 191