Income Taxes - Significant Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 42,872	$ 31,744
Research and development and orphan drug credits	6,212	4,615
Accrued expenses	519	332
Intangibles	60	69
Operating lease liabilities	545
Other, net	322	54
Total deferred tax assets	50,530	36,814
Operating lease right-of-use assets	(527)
Property and equipment, net	(28)	(18)
Total deferred tax liabilities	(555)	(18)
Net deferred tax assets before valuation allowance	49,975	36,796
Valuation allowance	(49,975)	(36,796)
Net deferred tax assets	$ 0	$ 0