Sanofi License and Collaboration Agreement - Schedule of Receivables and Related Prepayments From Collaboration Partner (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Prepayment from collaboration partner for mavacamten, Balance at Beginning of Period	$ 12,973
Prepayment from collaboration partner for mavacamten, Balance at End of Period	2,256
Collaborative Agreement | Sanofi (Aventis Inc.)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Receivable from collaboration partner, Balance at Beginning of Period	0	$ 1,013
Deductions	0	(1,013)
Receivable from collaboration partner, Balance at End of Period	0	0
Prepayment from collaboration partner for mavacamten, Balance at Beginning of Period	12,973	4,432
Payments received from Sanofi	7,777	11,809
Actual expenses incurred	(18,494)	(7,126)
Prepayment from collaboration partner for mavacamten, Balance at End of Period	$ 2,256	$ 9,115