Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Provision or benefit from income taxes	$ 0	$ 0	$ 0
Corporate income tax rates	21.00%
Orphan drug credit of qualifying expenditures percentage	25.00%
Effect of tax cuts and jobs act on income tax	$ 0	7,869,000
Net operating loss carryforwards	96,600,000	58,000,000
Unrecognized tax benefit	$ 0	0
Earliest Tax Year
Income Taxes [Line Items]
Net operating loss carryforwards, expiration year	2033
Tax credit carryforwards, expiration year	2033
Tax Cuts and Jobs Act of 2017
Income Taxes [Line Items]
Increase in valuation allowance	$ 9,800,000	2,700,000	9,400,000
Federal
Income Taxes [Line Items]
Tax credit carryforwards	7,300,000	5,600,000
State
Income Taxes [Line Items]
Tax credit carryforwards	400,000	1,000,000
PATH Act of 2015
Income Taxes [Line Items]
Converted research tax credit to offset payroll tax liabilities	0	500,000	$ 0
PATH Act of 2015 | Prepaid Expenses and Other Current Assets
Income Taxes [Line Items]
Payroll tax credit receivable	300,000	400,000
PATH Act of 2015 | Other Noncurrent Assets
Income Taxes [Line Items]
Payroll tax credit receivable	$ 0	$ 100,000
Maximum
Income Taxes [Line Items]
Corporate income tax rates		35.00%
Orphan drug credit of qualifying expenditures percentage		50.00%