| Document and Entity Information | Sep. 24, 2019 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | MOODYS CORP /DE/ |
| Entity Address, State or Province | NY |
| Amendment Flag | false |
| Entity Central Index Key | 0001059556 |
| Document Type | 8-K |
| Document Period End Date | Sep. 24, 2019 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-14037 |
| Entity Tax Identification Number | 13-3998945 |
| Entity Address, Address Line One | 7 World Trade Center at 250 Greenwich Street |
| Entity Address, City or Town | New York |
| Entity Address, Postal Zip Code | 10007 |
| City Area Code | (212) |
| Local Phone Number | 553-0300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Stock, par value $0.01 per share |
| Trading Symbol | MCO |
| Security Exchange Name | NYSE |
| Two Thousand Fifteen Senior Notes [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 1.75% Senior Notes Due 2027 |
| Trading Symbol | MCO 27 |
| Security Exchange Name | NYSE |
