Collaboration and License Agreement - Additional Information (Details) - USD ($)		1 Months Ended	3 Months Ended									12 Months Ended			24 Months Ended	43 Months Ended	53 Months Ended
	Aug. 31, 2014	Jan. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration and license revenue												$ 33,558,000	$ 11,442,000	$ 41,971,000
Collaboration and license revenue			$ 12,400,000	$ 9,188,000	$ 6,639,000	$ 5,331,000	$ 2,954,000	$ 3,077,000	$ 3,001,000	$ 2,410,000			11,442,000	41,971,000
Deferred revenue							33,558,000				$ 45,000,000		33,558,000	45,000,000
Short-term receivable from collaboration partner							1,013,000				45,000,000		1,013,000	45,000,000
Prepayment from collaboration partner												(8,541,000)	(4,432,000)
Collaborative Agreement | Sanofi (Aventis Inc.)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment received for research and development																	$ 105,000,000
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
Up-front equity investment	10,000,000
Additional equity investments received																$ 26,500,000
Eligible to receive one-time, non-refundable contingent payment	25,000,000
Project non-refundable continuation payment	45,000,000
Milestone payment received	25,000,000										$ 25,000,000
Continuation payment, amount received		$ 45,000,000											45,000,000	35,000,000
Upfront cash payment	34,300,000
Net fair value allocated to equity	700,000
Variable consideration	25,000,000
Fixed fee															$ 45,000,000
Transaction price for extended term															45,000,000
Collaboration and license revenue												33,600,000	11,400,000	42,000,000
Deferred revenue			0									0			0		0
Short-term receivable from collaboration partner			$ 0				$ 1,013,000					0	1,013,000		$ 0		$ 0
Prepayment from collaboration partner												13,000,000	4,400,000
Reduction in research and development expenses due to RPP reimbursements												$ 23,100,000	$ 7,300,000	$ 0
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
Additional research and development funding for collaboration	$ 15,000,000