Reconciliation Between Effective Tax Rate and Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal income tax rate	21.00%	21.00%	21.00%
State taxes	4.00%	6.30%	12.30%
Research and development and other tax credits	6.60%	3.30%	3.10%
Valuation allowance	(21.30%)	(16.80%)	(45.50%)
Permanent differences	2.20%	1.80%	6.90%
Sarepta International C.V. return to provision			(0.10%)
Basis difference in subsidiary		(8.40%)
Foreign rate differential	(12.90%)	(7.40%)	(0.90%)
Other	0.20%		3.40%
Effective tax rate	(0.20%)	(0.20%)	0.20%