| Document and Entity Information | Sep. 30, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 30, 2021 |
| Entity Registrant Name | CYCLERION THERAPEUTICS, INC. |
| Entity Incorporation, State or Country Code | MA |
| Entity File Number | 001-38787 |
| Entity Tax Identification Number | 83-1895370 |
| Entity Address, Address Line One | 245 First Street, 18th Floor |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02142 |
| City Area Code | 857 |
| Local Phone Number | 327-8778 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0001755237 |
| Title of 12(b) Security | Common Stock, no par value |
| Trading Symbol | CYCN |
| Security Exchange Name | NASDAQ |
