Income Taxes - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Feb. 28, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Net change in valuation allowance for deferred tax assets		$ 119,500,000	$ 164,800,000
Operating loss carryforwards, valuation allowance		7,900,000
Net operating loss carried forwards		304,033,000	212,342,000
Accrual for interest or penalties		$ 0	$ 0
Recognized interest and/or penalties				$ 0	$ 0
Minimum [Member]
Income Taxes [Line Items]
Operating loss carryforwards, ending of expiration year		2020
Maximum [Member]
Income Taxes [Line Items]
Operating loss carryforwards, ending of expiration year		2039
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		$ 1,166,200,000
Net operating loss carryforwards subject to expiration		579,900,000
Net operating loss carried forwards		586,300,000
State [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		859,000,000.0
Net operating loss carryforwards subject to expiration		807,700,000
Net operating loss carried forwards		51,300,000
Foreign [Member] | Sarepta International Holdings GmbH [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		$ 8,000,000.0
Research and Development Credits [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Description		These federal and state research and development credits begin to expire between 2020 and 2039 and between 2020 and 2034, respectively.
Research and Development Credits [Member] | Federal [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Amount		$ 74,700,000
Research and Development Credits [Member] | Federal [Member] | Minimum [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period		2020
Research and Development Credits [Member] | Federal [Member] | Maximum [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period		2039
Research and Development Credits [Member] | State [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Amount		$ 34,100,000
Research and Development Credits [Member] | State [Member] | Minimum [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period		2020
Research and Development Credits [Member] | State [Member] | Maximum [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period		2034
Warrant Agreement | Myonexus Therapeutics, Inc. [Member]
Income Taxes [Line Items]
Accumulated costs related to acquisition	$ 253,700,000
Up-front and milestone payments	85,000,000.0
Exercise option	$ 168,700,000