Cumulative Effect of the Change in Beginning Financial Position for the adoption of Revenue Accounting Standard (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 1,314.5			$ 1,071.5	$ 3,280.9		$ 2,051.5
Short-term investments	101.2			111.8
Accounts receivable, net of allowances	1,137.1			1,147.2
Other current assets	286.9			250.1
Assets Current	2,839.7			2,580.6
Property and equipment, net	312.2			325.1
Goodwill	3,662.7			3,753.2			1,023.6
Intangible assets, net	1,541.4			1,631.6
Deferred tax assets, net	136.6			143.8
Other assets	259.0			159.9
Total Assets	8,751.6			8,594.2
Current liabilities:
Accounts payable and accrued liabilities	593.6			750.3
Commercial paper	89.9			129.9
Current portion of long-term debt	299.9			299.5
Deferred revenue	860.5			883.6
Total current liabilities	1,843.9			2,063.3
Non-current portion of deferred revenue	126.0			140.0
Long-term debt	4,934.2			5,111.1
Deferred tax liabilities, net	363.2			341.6
Unrecognized tax benefits	398.3			389.1
Other liabilities	625.9			664.0
Total liabilities	8,291.5			8,709.1
Shareholders' (deficit) equity
Common stock	3.4			3.4
Capital surplus	538.6			528.6
Retained earnings	8,204.6			7,465.4
Treasury stock, value	(8,198.9)			(8,152.9)
Accumulated other comprehensive loss	(304.8)	$ (51.9)		(172.2)	$ (309.7)	$ (357.3)	(364.9)
Total Moody's shareholders' deficit	242.9			(327.7)
Noncontrolling interests	217.2			212.8			$ 197.7
Total shareholders' deficit	460.1			(114.9)
Total liabilities and shareholders' (deficit) equity	8,751.6			$ 8,594.2
New Revenue Accounting Standard [Member]
Current assets:
Cash and cash equivalents			$ 1,071.5
Short-term investments			111.8
Accounts receivable, net of allowances			1,164.0
Other current assets			283.0
Assets Current			2,630.3
Property and equipment, net			325.1
Goodwill			3,753.2
Intangible assets, net			1,631.6
Deferred tax assets, net			143.8
Other assets			231.2
Total Assets			8,715.2
Current liabilities:
Accounts payable and accrued liabilities			749.5
Commercial paper			129.9
Current portion of long-term debt			299.5
Deferred revenue			814.3
Total current liabilities			1,993.2
Non-current portion of deferred revenue			132.0
Long-term debt			5,111.1
Deferred tax liabilities, net			384.3
Unrecognized tax benefits			389.1
Other liabilities			664.3
Total liabilities			8,674.0
Shareholders' (deficit) equity
Common stock			3.4
Capital surplus			528.6
Retained earnings			7,621.5
Treasury stock, value			(8,152.9)
Accumulated other comprehensive loss			(172.2)
Total Moody's shareholders' deficit			(171.6)
Noncontrolling interests			212.8
Total shareholders' deficit			41.2
Total liabilities and shareholders' (deficit) equity			8,715.2
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | New Revenue Accounting Standard [Member]
Current assets:
Accounts receivable, net of allowances	14.2		16.8
Other current assets	22.7		32.9
Assets Current	36.9		49.7
Deferred tax assets, net	0.1
Other assets	70.0		71.3
Total Assets	107.0		121.0
Current liabilities:
Accounts payable and accrued liabilities	0.2		(0.8)
Deferred revenue	(70.3)		(69.3)
Total current liabilities	(70.1)		(70.1)
Non-current portion of deferred revenue	(7.6)		(8.0)
Deferred tax liabilities, net	28.1		42.7
Other liabilities	0.3		0.3
Total liabilities	(49.3)		(35.1)
Shareholders' (deficit) equity
Retained earnings	156.3		156.1
Total Moody's shareholders' deficit	156.3		156.1
Total shareholders' deficit	156.3		156.1
Total liabilities and shareholders' (deficit) equity	$ 107.0		$ 121.0