| Cover | Apr. 02, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Apr. 2, 2020 |
| Current Fiscal Year End Date | --12-31 |
| Entity File Number | 001-37926 |
| Entity Registrant Name | RA PHARMACEUTICALS, INC. |
| Entity Central Index Key | 0001481512 |
| Entity Tax Identification Number | 26-2908274 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 87 Cambridgepark Drive |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02140 |
| City Area Code | 617 |
| Local Phone Number | 401-4060 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value per share |
| Trading Symbol | RARX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
