Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Provision (benefit) for income taxes	$ (86,525)	$ 138	$ 136
Domestic	28,794	25,123	(15,693)
Foreign	234	223	262
Income tax benefit associated with the loss before income taxes	0	0	$ 0
Net deferred tax liability		15
Net operating loss carryforwards	52,034	55,742
Deferred tax assets related to U.S. federal research and development credits	$ 12,000
Net operating loss carryforwards beginning expiration year	2031
U.S. research and development credit beginning expiration year	2024
Orphan drug credit beginning expiration year	2030
Deferred tax assets relating to U.S. state NOL carryforwards	$ 8,700
Unrecognized tax benefits that would impact effective tax rate	9,700
U.S. Federal
Income Taxes [Line Items]
Net operating loss carryforwards	43,300
Orphan Drug
Income Taxes [Line Items]
Net deferred tax assets	$ 24,000
District of Columbia
Income Taxes [Line Items]
Net operating loss carryforwards beginning expiration year	2031
Other Non-current Liabilities | Maximum
Income Taxes [Line Items]
Net deferred tax liability		$ 100