Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Provision for income taxes		$ 0
Federal operating loss carryforwards		482,900,000
State operating loss carryforwards		$ 479,900,000
Federal and state net operating loss carryforwards expiration year		2031
Federal orphan drug tax credit carry forwards		$ 40,000,000
Tax credit carry forwards expiration period		2034
U.S. corporate tax rate		34.00%	34.00%	34.00%
Reduction to deferred tax asset		$ 66,400,000
Change in amount of deferred tax assets		70,400,000
Valuation allowance		205,441,000	$ 135,072,000
Accrued interest or penalties related to uncertain tax positions		0	0
Uncertain tax positions		0
ASU 2016-09 [Member]
Income Tax Disclosure [Line Items]
Increase in deferred tax assets associated with net operating losses			8,000,000
Increase in deferred tax assets associated with federal and state research credits			$ 1,400,000
ASU 2015-17 [Member] | ASU Early Adoption [Member]
Income Tax Disclosure [Line Items]
Decrease in deferred tax liability current				$ (600,000)
Increase in deferred tax liability non-current				$ 600,000
Scenario Forecast [Member]
Income Tax Disclosure [Line Items]
U.S. corporate tax rate	21.00%
Domestic Tax Authority [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards		$ 12,700,000
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		$ 2,300,000
Research and development tax credits carryforwards expiration year		2027