Income Taxes - Additional Information (Details) - USD ($)			3 Months Ended	12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Income tax benefits			$ 0	$ 0	$ 0
Federal statutory income tax rate	21.00%	35.00%		21.00%	21.00%	34.00%
Percentage of net operating loss carry forward deductible from current year taxable income					80.00%
Research and orphan drug credit carryforwards				$ 25,267,000	$ 13,457,000
Unrecognized tax benefits				$ 0
Income tax examination, description				There are currently no pending income tax examinations. The Company's tax years that are open under statute are from October 2, 2017 to the present.
Accrued interest or penalties related to uncertain tax positions				$ 0	0
Interest Expense
Income Tax Disclosure [Line Items]
Accrued interest or penalties related to uncertain tax positions				0	$ 0
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards				324,200,000
Net operating loss carryforwards, subject to expiration				$ 14,500,000
Tax credit carryforwards expiration year				2037
Net operating loss carryforwards, carried forward indefinitely				$ 309,700,000
Research and orphan drug credit carryforwards				23,100,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards				$ 288,600,000
Tax credit carryforwards expiration year				2032	2032
Net operating loss carryforwards expiration year				2037
Research and orphan drug credit carryforwards				$ 2,800,000