| Document and Entity Information - shares | 3 Months Ended | |
| Mar. 31, 2021 | Apr. 27, 2021 | |
| Cover [Abstract] | ||
| Document Type | 10-Q/A | |
| Amendment Flag | false | |
| Document Quarterly Report | true | |
| Document Period End Date | Mar. 31, 2021 | |
| Current Fiscal Year End Date | --12-31 | |
| Document Transition Report | false | |
| 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 | |
| Entity Filer Category | Non-accelerated Filer | |
| Entity Current Reporting Status | Yes | |
| Entity Interactive Data Current | Yes | |
| Entity Emerging Growth Company | true | |
| Entity Ex Transition Period | false | |
| Entity Shell Company | false | |
| Entity Small Business | true | |
| Entity Common Stock, Shares Outstanding | 34,134,466 | |
| Document Fiscal Year Focus | 2021 | |
| Document Fiscal Period Focus | Q1 | |
| Entity Central Index Key | 0001755237 | |
| Title of 12(b) Security | Common Stock, no par value | |
| Trading Symbol | CYCN | |
| Security Exchange Name | NASDAQ |
