Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate

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

	Year Ended December 31,
	2019			2018			2017
Tax due at statutory rate		21.0	%		21.0	%		34.0	%
State taxes, net of federal		2.8			6.0			5.1
Stock-based compensation		1.4			2.5			6.4
Foreign rate differential		(1.6	)		(4.3	)		(3.2	)
Federal and state credits		2.4			2.5			7.2
Change in valuation allowance		(25.8	)		(27.6	)		(22.5	)
Other		0.5			(0.1	)		—
Federal and state rate change		(0.7	)		—			(24.6	)
Research and orphan drug credit addback		—			—			(2.4	)
		0.0	%		0.0	%		0.0	%

Summary of Significant Components of Company's Net Deferred Tax Asset

Significant components of the Company’s net deferred tax asset at December 31, 2019 and 2018 are as follows:

	December 31,
	2019			2018
	(in thousands)
Net operating losses	$	348,848		$	206,460
Capitalized start-up costs		982			1,242
Tax credit carryforwards		80,088			63,653
Accrued expenses		4,055			4,498
Depreciation and amortization		2,002			1,001
Stock options		46,750			30,925
Right of use asset		(8,059	)		—
Right of use liability		8,852			—
Other		(246	)		568
Total net deferred tax asset before valuation    allowance		483,272			308,347
Valuation allowance		(483,272	)		(308,347	)
Net deferred tax asset	$	—		$	—