| Document and Entity Information | Jan. 03, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | MyoKardia Inc |
| Amendment Flag | false |
| Entity Central Index Key | 0001552451 |
| Document Type | 8-K |
| Document Period End Date | Jan. 3, 2020 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-37609 |
| Entity Tax Identification Number | 44-5500552 |
| Entity Address, Address Line One | 333 Allerton Ave. |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | (650) |
| Local Phone Number | 741-0900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock |
| Trading Symbol | MYOK |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
