Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Current tax expense	$ 0
Deferred tax expense	0
Federal operating loss carryforwards	1,400,000,000
State operating loss carryforwards	$ 1,000,000,000.0
Federal net operating loss carryforwards expiration year	2031
State net operating loss carryforwards expiration year	2030
Federal orphan drug tax credit carry forwards	$ 40,000,000.0
Tax credit carry forwards expiration period	2034
U.S. corporate tax rate	21.00%	21.00%	34.00%
Reduction to deferred tax asset			$ 66,400,000
Change in amount of deferred tax assets	$ 174,900,000
Valuation allowance	483,272,000	$ 308,347,000
Unrecognized tax benefits	0	0	$ 0
Accrued interest or penalties related to uncertain tax positions	0	$ 0
Uncertain tax positions	0
Domestic Tax Authority [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards	$ 34,200,000
Research and development tax credits carryforwards expiration year	2031
State and Local Jurisdiction [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards	$ 7,400,000
Research and development tax credits carryforwards expiration year	2027