Financial Information by Segment (Parenthetical) (Detail) - Compensation-Retirement Benefits Accounting Standard [Member] $ in Millions	3 Months Ended
Mar. 31, 2017 USD ($)
Moodys Investors Service [Member]
Segment Reporting Information [Line Items]
Reduction in Operating, SG&A	$ 2.1
Moodys Analytics [Member]
Segment Reporting Information [Line Items]
Reduction in Operating, SG&A	$ 1.2

The amount of net period pension costs which were reclassed from either Operating or SG&A expenses to non-operating (expense) income, net within the statement of operations, as required by ASU 2017-07.

No definition available.

Name:	mco_ReductionInDirectOperatingCostsSellingGeneralAndAdministrativeExpense
Namespace Prefix:	mco_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration