Cumulative Effect of the Change in Beginning Financial Position for the adoption of Revenue Accounting Standard (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 1,277.3		$ 1,071.5	$ 2,129.6	$ 2,051.5
Short-term investments	100.5		111.8
Accounts receivable, net of allowances	1,207.4		1,147.2
Other current assets	233.6		250.1
Assets Current	2,818.8		2,580.6
Property and equipment, net	321.6		325.1
Goodwill	3,831.5		3,753.2		1,023.6
Intangible assets, net	1,641.6		1,631.6
Deferred tax assets, net	142.5		143.8
Other assets	258.0		159.9
Total Assets	9,014.0		8,594.2
Current liabilities:
Accounts payable and accrued liabilities	532.6		750.3
Commercial paper	89.9		129.9
Current portion of long-term debt	299.7		299.5
Deferred revenue	998.7		883.6
Total current liabilities	1,920.9		2,063.3
Non-current portion of deferred revenue	127.8		140.0
Long-term debt	5,118.0		5,111.1
Deferred tax liabilities, net	382.2		341.6
Unrecognized tax benefits	490.9		389.1
Other liabilities	554.2		664.0
Total liabilities	8,594.0		8,709.1
Shareholders' (deficit) equity
Common stock	3.4		3.4
Capital surplus	506.6		528.6
Retained earnings	7,913.0		7,465.4
Treasury stock, value	(8,171.4)		(8,152.9)
Accumulated other comprehensive loss	(51.9)		(172.2)	$ (357.3)	$ (364.9)
Total Moody's shareholders' deficit	199.7		(327.7)
Noncontrolling interests	220.3		212.8
Total shareholders' deficit	420.0		(114.9)
Total liabilities and shareholders' (deficit) equity	9,014.0		$ 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	24.4	16.8
Other current assets	21.2	32.9
Assets Current	45.6	49.7
Deferred tax assets, net	(0.1)
Other assets	75.2	71.3
Total Assets	120.7	121.0
Current liabilities:
Accounts payable and accrued liabilities	0.1	(0.8)
Deferred revenue	(66.9)	(69.3)
Total current liabilities	(66.8)	(70.1)
Non-current portion of deferred revenue	(7.2)	(8.0)
Deferred tax liabilities, net	33.5	42.7
Other liabilities	0.3	0.3
Total liabilities	(40.2)	(35.1)
Shareholders' (deficit) equity
Retained earnings	160.9	156.1
Total Moody's shareholders' deficit	160.9	156.1
Total shareholders' deficit	160.9	156.1
Total liabilities and shareholders' (deficit) equity	$ 120.7	$ 121.0