Reconciliation Between Effective Tax Rate and Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal income tax rate	21.00%	21.00%	34.00%
State taxes	6.30%	12.30%	(27.70%)
Research and development and other tax credits	3.30%	3.10%	8.50%
Valuation allowance	(16.80%)	(45.50%)	(93.20%)
Permanent differences	1.80%	6.90%	6.40%
Sarepta International C.V. return to provision		(0.10%)	62.10%
Impact of tax reform, net of valuation allowance			5.90%
Basis difference in subsidiary	(8.40%)
Foreign rate differential	(7.40%)	(0.90%)	(0.60%)
Other		3.40%	0.40%
Effective tax rate	(0.20%)	0.20%	(4.20%)