CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Capital Surplus	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Total Moody's Shareholders' Equity (Deficit)	Noncontrolling 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
Net income	376.8			372.9			372.9	3.9
Dividends	(85.1)			(83.7)			(83.7)	(1.4)
Stock-based compensation	35.2		35.2				35.2
Shares issued for stock-based compensation plans, net	(32.3)		(57.2)		$ 24.9		(32.3)
Shares issued for stock-based compensation plans, net (in shares)					1.2
Treasury shares repurchased, shares					(0.3)
Treasury shares repurchased	(43.4)				$ (43.4)		(43.4)
Currency translation adjustment	125.2					120.2	120.2	5.0
Amortization of actuarial losses and prior service costs included in net income - Net of Tax	1.0					1.0	1.0
Net realized and unrealized gain on cash flow hedges (net of tax of $1.1 million)	1.4					1.4	1.4
Ending Balance at Mar. 31, 2018	420.0	$ 3.4	506.6	7,913.0	$ (8,171.4)	(51.9)	199.7	220.3
Ending Balance (in shares) at Mar. 31, 2018		342.9			(151.0)
Adoption of new ASU | New Revenue Accounting Standard [Member]	156.1			156.1			156.1
Adoption of new ASU | Accounting Standards Update 2016-01 [Member]				2.3		(2.3)
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
Net income	373.6			372.9			372.9	0.7
Dividends	(93.7)			(93.5)			(93.5)	(0.2)
Cash paid for ASR contract relating to shares retained by counterparty until final settlement	(125.3)		(125.3)				(125.3)
Stock-based compensation	35.8		35.8				35.8
Shares issued for stock-based compensation plans, net	(59.6)		(66.3)		$ 6.7		(59.6)
Shares issued for stock-based compensation plans, net (in shares)					1.0
Purchase of noncontrolling interest	(12.3)		(9.4)				(9.4)	(2.9)
Treasury shares repurchased, shares					(2.7)
Treasury shares repurchased	(448.2)				$ (448.2)		(448.2)
Currency translation adjustment	(3.2)					(10.8)	(10.8)	7.6
Net actuarial losses and prior service costs - Net of Tax	0.8					0.8	0.8
Amortization of actuarial losses and prior service costs included in net income - Net of Tax	0.6					0.6	0.6
Ending Balance at Mar. 31, 2019	$ 325.0	$ 3.4	$ 435.7	8,893.6	$ (8,754.0)	(455.5)	$ 123.2	$ 201.8
Ending Balance (in shares) at Mar. 31, 2019		342.9			(153.3)
Adoption of new ASU | Accounting Standards Update 2018-02 [Member]				$ 19.8		$ (19.8)