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
Beginning Balance (in shares) at Dec. 31, 2017		342.9			151.9
Beginning Balance at Dec. 31, 2017	$ (114.9)	$ 3.4	$ 528.6	$ 7,465.4	$ (8,152.9)	$ (172.2)	$ (327.7)	$ 212.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	1,066.7			1,059.3			1,059.3	7.4
Dividends	(258.0)			(254.0)			(254.0)	(4.0)
Stock-based compensation	100.2		100.2				100.2
Shares issued for stock-based compensation plans at average cost, net	(19.1)		(59.1)		$ 40.0		(19.1)
Shares issued for stock-based compensation plans at average cost, net (in shares)					1.5
Treasury shares repurchased	(147.2)				$ (147.2)		(147.2)
Treasury shares repurchased (in shares)					(0.9)
Currency translation adjustment and net gain on net investment hedges	(182.6)					(172.4)	(172.4)	(10.2)
Net actuarial gains (losses) and prior service costs	1.2					1.2	1.2
Amortization of prior service costs and actuarial losses	2.5					2.5	2.5
Net realized gain (loss) on cash flow hedges	1.2					1.2	1.2
Ending Balance (in shares) at Sep. 30, 2018		342.9			151.3
Ending Balance at Sep. 30, 2018	606.1	$ 3.4	569.7	8,429.1	$ (8,260.1)	(342.0)	400.1	206.0
Beginning Balance (in shares) at Jun. 30, 2018		342.9			151.0
Beginning Balance at Jun. 30, 2018	460.1	$ 3.4	538.6	8,204.6	$ (8,198.9)	(304.8)	242.9	217.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	312.0			310.2			310.2	1.8
Dividends	(88.3)			(85.7)			(85.7)	(2.6)
Stock-based compensation	30.7		30.7				30.7
Shares issued for stock-based compensation plans at average cost, net	5.4		0.4		$ 5.0		5.4
Shares issued for stock-based compensation plans at average cost, net (in shares)					0.1
Treasury shares repurchased	(66.2)				$ (66.2)		(66.2)
Treasury shares repurchased (in shares)					(0.4)
Currency translation adjustment and net gain on net investment hedges	(48.3)					(37.9)	(37.9)	(10.4)
Amortization of prior service costs and actuarial losses	0.8					0.8	0.8
Net realized gain (loss) on cash flow hedges	(0.1)					(0.1)	(0.1)
Ending Balance (in shares) at Sep. 30, 2018		342.9			151.3
Ending Balance at Sep. 30, 2018	606.1	$ 3.4	569.7	8,429.1	$ (8,260.1)	(342.0)	400.1	206.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of new ASU | New Revenue Accounting Standard	156.1			156.1			156.1
Adoption of new ASU | Financial Instruments - Overall (ASU 2016-01)	0.0			2.3		(2.3)	0.0
Beginning Balance (in shares) at Dec. 31, 2018		342.9			151.6
Beginning Balance at Dec. 31, 2018	656.5	$ 3.4	600.9	8,594.4	$ (8,312.5)	(426.3)	459.9	196.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	1,068.0			1,062.6			1,062.6	5.4
Dividends	(286.3)			(285.2)			(285.2)	(1.1)
Stock-based compensation	103.5		103.5				103.5
Shares issued for stock-based compensation plans at average cost, net	(39.6)		(71.4)		$ 31.8		(39.6)
Shares issued for stock-based compensation plans at average cost, net (in shares)					1.5
Purchase of noncontrolling interest	(12.3)		(9.4)				(9.4)	(2.9)
Non-controlling interest resulting from majority acquisition of Vigeo Eiris	17.4							17.4
Treasury shares repurchased	(728.0)		(15.7)		$ (712.3)		(728.0)
Treasury shares repurchased (in shares)					(4.1)
Currency translation adjustment and net gain on net investment hedges	(82.5)					(91.8)	(91.8)	9.3
Net actuarial gains (losses) and prior service costs	(1.3)					(1.3)	(1.3)
Amortization of prior service costs and actuarial losses	1.8					1.8	1.8
Net realized gain (loss) on cash flow hedges	0.1					0.1	0.1
Ending Balance (in shares) at Sep. 30, 2019		342.9			154.2
Ending Balance at Sep. 30, 2019	697.3	$ 3.4	607.9	9,391.6	$ (8,993.0)	(537.3)	472.6	224.7
Beginning Balance (in shares) at Jun. 30, 2019		342.9			153.7
Beginning Balance at Jun. 30, 2019	574.7	$ 3.4	574.5	9,107.7	$ (8,886.7)	(446.1)	352.8	221.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	382.4			379.4			379.4	3.0
Dividends	(95.8)			(95.5)			(95.5)	(0.3)
Stock-based compensation	33.4		33.4				33.4
Shares issued for stock-based compensation plans at average cost, net	6.5		0.0		$ 6.5		6.5
Shares issued for stock-based compensation plans at average cost, net (in shares)					0.2
Treasury shares repurchased	(112.8)		0.0		$ (112.8)		(112.8)
Treasury shares repurchased (in shares)					(0.7)
Currency translation adjustment and net gain on net investment hedges	(91.7)					(91.8)	(91.8)	0.1
Amortization of prior service costs and actuarial losses	0.6					0.6	0.6
Ending Balance (in shares) at Sep. 30, 2019		342.9			154.2
Ending Balance at Sep. 30, 2019	$ 697.3	$ 3.4	$ 607.9	$ 9,391.6	$ (8,993.0)	$ (537.3)	$ 472.6	$ 224.7