Organization	12 Months Ended
Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization MyoKardia, Inc. (the “Company”) is a clinical-stage biopharmaceutical company pioneering a precision medicine approach to discover, develop and commercialize targeted therapies for the treatment of serious and neglected rare cardiovascular diseases. The Company’s initial focus is on the treatment of heritable cardiomyopathies, a group of rare, genetically driven forms of heart failure that result from biomechanical defects in cardiac muscle contraction. The Company has used its precision medicine platform to generate a robust pipeline of therapeutic programs. MyoKardia’s most advanced program, mavacamten, is in four clinical trials, including a pivotal Phase 3 study for the treatment of hypertrophic cardiomyopathy. A second clinical-stage candidate, MYK-491, is in a Phase 2a multiple-ascending dose study in patients with stable systolic heart failure. The Company was incorporated on June 8, 2012 in Delaware and its corporate headquarters and operations are located in South San Francisco, California. Liquidity The Company has incurred significant operating losses since inception and has an accumulated deficit of $202.6 million as of December 31, 2018. The Company has relied on its ability to fund its operations through private and public equity financings and to a lesser extent, through a license and collaboration arrangement with a collaboration partner, Sanofi S.A. (“Sanofi”) via its subsidiary, Aventis, Inc. As discussed further in Note 3, the collaboration agreement ended on December 31, 2018 and the Company will no longer record revenues from Sanofi in 2019 nor receive reimbursements of research and development expenses after June 30, 2019. The Company has not yet received regulatory approval to commercialize or sell any product and does not have customers. Management expects operating losses and negative operating cash flows to continue for the foreseeable future. As the Company continues to incur losses, a transition to profitability is dependent upon the successful development, approval, and commercialization of the Company’s products and product candidates and the achievement of a level of revenues adequate to support its cost structure. The Company’s ultimate success depends on the outcome of its research and development activities. The Company expects to incur additional losses and negative cash flows for the foreseeable future and it anticipates the need to raise additional capital to fully implement its business plan. The Company intends to raise such capital through the issuance of additional equity, debt and/or strategic alliances with partner companies. There is no assurance that such financing will be available on terms acceptable to the Company, if at all. On March 8, 2018, the Company filed a Shelf Registration Statement (“2018 Shelf Registration Statement”) covering the potential offering, issuance, and sale of an indeterminate amount of common stock, preferred stock, debt securities, warrants and/or units. In May 2018, the Company completed a follow-on offering under the 2018 Shelf Registration Statement pursuant to which the Company issued 3,750,000 shares of common stock at a price of $49.00 per share. In June 2018, the Company sold an additional 211,147 shares of common stock directly to the underwriters when they partially exercised their over-allotment option at the price of $49.00 per share. In the quarter ended June 30, 2018, the Company received net proceeds totaling $181.9 million from the offering, net of underwriting discounts and commissions and offering expenses, which includes $9.7 million received due to the underwriters’ partial exercise of their over-allotment option. As of December 31, 2018, the Company had $394.8 million of cash, cash equivalents and short and long-term investments which management believes will be sufficient to meet the Company’s anticipated operating and capital expenditure requirements for the twelve months following the date of this Form 10-K. Management’s belief with respect to its ability to fund operations is based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional funding.

Organization

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

1. Organization

MyoKardia, Inc. (the “Company”) is a clinical-stage biopharmaceutical company pioneering a precision medicine approach to discover, develop and commercialize targeted therapies for the treatment of serious and neglected rare cardiovascular diseases. The Company’s initial focus is on the treatment of heritable cardiomyopathies, a group of rare, genetically driven forms of heart failure that result from biomechanical defects in cardiac muscle contraction. The Company has used its precision medicine platform to generate a robust pipeline of therapeutic programs. MyoKardia’s most advanced program, mavacamten, is in four clinical trials, including a pivotal Phase 3 study for the treatment of hypertrophic cardiomyopathy. A second clinical-stage candidate, MYK-491, is in a Phase 2a multiple-ascending dose study in patients with stable systolic heart failure. The Company was incorporated on June 8, 2012 in Delaware and its corporate headquarters and operations are located in South San Francisco, California.

Liquidity

The Company has incurred significant operating losses since inception and has an accumulated deficit of $202.6 million as of December 31, 2018. The Company has relied on its ability to fund its operations through private and public equity financings and to a lesser extent, through a license and collaboration arrangement with a collaboration partner, Sanofi S.A. (“Sanofi”) via its subsidiary, Aventis, Inc. As discussed further in Note 3, the collaboration agreement ended on December 31, 2018 and the Company will no longer record revenues from Sanofi in 2019 nor receive reimbursements of research and development expenses after June 30, 2019. The Company has not yet received regulatory approval to commercialize or sell any product and does not have customers. Management expects operating losses and negative operating cash flows to continue for the foreseeable future. As the Company continues to incur losses, a transition to profitability is dependent upon the successful development, approval, and commercialization of the Company’s products and product candidates and the achievement of a level of revenues adequate to support its cost structure. The Company’s ultimate success depends on the outcome of its research and development activities. The Company expects to incur additional losses and negative cash flows for the foreseeable future and it anticipates the need to raise additional capital to fully implement its business plan. The Company intends to raise such capital through the issuance of additional equity, debt and/or strategic alliances with partner companies. There is no assurance that such financing will be available on terms acceptable to the Company, if at all.

On March 8, 2018, the Company filed a Shelf Registration Statement (“2018 Shelf Registration Statement”) covering the potential offering, issuance, and sale of an indeterminate amount of common stock, preferred stock, debt securities, warrants and/or units. In May 2018, the Company completed a follow-on offering under the 2018 Shelf Registration Statement pursuant to which the Company issued 3,750,000 shares of common stock at a price of $49.00 per share. In June 2018, the Company sold an additional 211,147 shares of common stock directly to the underwriters when they partially exercised their over-allotment option at the price of $49.00 per share. In the quarter ended June 30, 2018, the Company received net proceeds totaling $181.9 million from the offering, net of underwriting discounts and commissions and offering expenses, which includes $9.7 million received due to the underwriters’ partial exercise of their over-allotment option.

As of December 31, 2018, the Company had $394.8 million of cash, cash equivalents and short and long-term investments which management believes will be sufficient to meet the Company’s anticipated operating and capital expenditure requirements for the twelve months following the date of this Form 10-K. Management’s belief with respect to its ability to fund operations is based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional funding.