Income Taxes - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Feb. 28, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Net change in valuation allowance for deferred tax assets		$ 117,000,000.0	$ 119,500,000
Operating loss carryforwards, valuation allowance		7,900,000
Net operating loss carried forwards		333,703,000	304,033,000
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			2021
Maximum [Member]
Income Taxes [Line Items]
Operating loss carryforwards, ending of expiration year			2040
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		1,298,300,000
Net operating loss carryforwards subject to expiration		581,300,000
Net operating loss carried forwards		717,000,000.0
State [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		872,300,000
Net operating loss carryforwards subject to expiration		819,900,000
Net operating loss carried forwards		52,400,000
Foreign [Member] | Sarepta International Holdings GmbH [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		11,400,000
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 2021 and 2040 and between 2021 and 2035, respectively.
Research and Development Credits [Member] | Federal [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Amount		111,900,000
Research and Development Credits [Member] | Federal [Member] | Minimum [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period			2021
Research and Development Credits [Member] | Federal [Member] | Maximum [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period			2040
Research and Development Credits [Member] | State [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Amount		$ 57,700,000
Research and Development Credits [Member] | State [Member] | Minimum [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period			2021
Research and Development Credits [Member] | State [Member] | Maximum [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period			2035
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