Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Reconciliation of U.S. Federal Statutory Income Tax Rate to Effective Income Tax Rate

A reconciliation of the U.S. federal statutory income tax rate to the Company's effective income tax rate is as follows:

	Year Ended December 31,
	2019		2018		2017
Federal statutory income tax rate	21.0	%	21.0	%	34.0	%
State taxes, net of federal benefit	7.0		6.1		1.6
Research and orphan drug credit	5.4		9.6		7.7
Research and orphan drug credit addback	—		—		(6.6)
Impact of change in tax status	—		—		(8.7)
Effect of federal tax law change	—		—		(6.1)
Permanent adjustments and other	3.8		0.7		(0.3)
Increase in deferred tax asset valuation allowance	(37.2)		(37.4)		(21.6)
Effective income tax rate	—	%	—	%	—	%

Schedule of Net Deferred Tax Assets

Net deferred tax assets consisted of the following:

	December 31,
(in thousands)	2019		2018
Deferred tax assets (liabilities):
Net operating loss carryforwards	$	85,632	$	28,906
Research and orphan drug credit carryforwards	25,267		13,457
Stock-based compensation	5,535		4,681
Accrued expenses	2,951		1,270
Operating lease liabilities	5,841		3,200
Operating lease assets	(5,515)		—
Property and equipment	—		(3,200)
Other	(64)		(123)
Total gross deferred tax assets	119,647		48,191
Valuation allowance	(119,647)		(48,191)
Net deferred tax assets	$	—	$	—

Schedule of Changes in Valuation Allowance for Deferred Tax Assets

The change in the valuation allowance was as follows:

	Year Ended December 31,
(in thousands)	2019		2018
Valuation allowance as of beginning of year	$	(48,191)	$	(10,835)
Net increases recorded to income tax provision	(71,456)		(37,356)
Valuation allowance as of end of year	$	(119,647)	$	(48,191)