Sanofi License and Collaboration Agreement - Additional Information (Details) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended		24 Months Ended	43 Months Ended	53 Months Ended
	Aug. 31, 2014	Jan. 31, 2017	Jun. 30, 2018	Dec. 31, 2016	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2018	Jul. 31, 2019	Jul. 18, 2019	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue			$ 6,639,000			$ 11,970,000
Deferred revenue			$ 21,588,000			21,588,000						$ 33,558,000
Contract assets					$ 0
Contract liabilities					0
Royalty Bearing Licenses | Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount Payable to reacquire research and development asset										$ 80,000,000
Sanofi (Aventis Inc.) | Royalty Rights | Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount Payable to reacquire research and development asset										80,000,000	$ 80,000,000
Collaborative Agreement | Sanofi (Aventis Inc.)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment received for research and development									$ 105,000,000.0
Collaboration and license revenue									$ 104,300,000
Type of Revenue [Extensible List]									myok:ResearchAndDevelopmentMember
Research and development payment received attributed to redeemable convertible preferred stock call option									$ 700,000
Upfront cash received under collaboration agreement	$ 35,000,000.0
Up-front equity investment	10,000,000.0
Additional equity investments received								$ 26,500,000
Eligible to receive one-time, non-refundable contingent payment	25,000,000.0
Project non-refundable continuation payment	45,000,000.0
Milestone payment received	25,000,000.0			$ 25,000,000.0
Continuation payment, amount received		$ 45,000,000.0
Upfront cash payment	34,300,000
Net fair value allocated to equity	700,000
Variable consideration	25,000,000.0
Fixed fee							$ 45,000,000.0
Transaction price for extended term							$ 45,000,000.0
Collaboration and license revenue					0	12,000,000.0
Deferred revenue					0
Reduction in research and development expenses due to RPP reimbursements					$ 18,500,000	$ 7,100,000
Collaborative Agreement | Sanofi (Aventis Inc.) | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Funding from approved in-kind research and clinical activities	45,000,000.0
Additional research and development funding for collaboration	$ 15,000,000.0
Termination Agreement [Member] | Sanofi (Aventis Inc.) | Royalty Rights | Subsequent Event
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount Payable to reacquire research and development asset										$ 80,000,000