Effect of Change in Income Statement due to adoption of Revenue Accounting Standard (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 1,126.7	$ 975.2
Expenses
Operating	314.9	275.3
Selling, general and administrative	271.1	220.7
Depreciation and amortization	49.1	32.5
Acquisition-Related Expenses	0.8
Total expenses	635.9	528.5
Operating income	490.8	446.7
Non-operating (expense) income, net
Interest income (expense), net	(50.7)	(47.4)
Other non-operating income (expense), net	1.0	(7.7)
Total non-operating (expense) income, net	(49.7)	4.6
Income before provisions for income taxes	441.1	451.3
Provision for income taxes	64.3	105.4
Net income	376.8	345.9
Less: Net income attributable to noncontrolling interests	3.9	0.3
Net income attributable to Moody's	$ 372.9	$ 345.6
Earnings per share attributable to Moody's common shareholders
Basic	$ 1.95	$ 1.81
Diluted	$ 1.92	$ 1.78
Weighted average number of shares outstanding
Basic	191.4	191.1
Diluted	194.5	194.3
Under Previous Accounting Guidance [Member] | New Revenue Accounting Standard [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 1,126.4
Expenses
Operating	315.5
Selling, general and administrative	271.5
Depreciation and amortization	49.1
Acquisition-Related Expenses	0.8
Total expenses	636.9
Operating income	489.5
Non-operating (expense) income, net
Interest income (expense), net	(50.7)
Other non-operating income (expense), net	1.0
Total non-operating (expense) income, net	(49.7)
Income before provisions for income taxes	439.8
Provision for income taxes	63.5
Net income	376.3
Less: Net income attributable to noncontrolling interests	3.9
Net income attributable to Moody's	$ 372.4
Earnings per share attributable to Moody's common shareholders
Basic	$ 1.95
Diluted	$ 1.91
Weighted average number of shares outstanding
Basic	191.4
Diluted	194.5
Effect of Change [Member] | New Revenue Accounting Standard [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 0.3
Expenses
Operating	(0.6)
Selling, general and administrative	(0.4)
Total expenses	(1.0)
Operating income	1.3
Non-operating (expense) income, net
Income before provisions for income taxes	1.3
Provision for income taxes	0.8
Net income	0.5
Net income attributable to Moody's	$ 0.5
Earnings per share attributable to Moody's common shareholders
Diluted	$ 0.01