Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018			2017			2016
Tax due at statutory rate		21.0	%		34.0	%		34.0	%
State taxes, net of federal		6.0			5.1			4.2
Stock-based compensation		2.5			6.4			(1.0	)
Foreign rate differential		(4.3	)		(3.2	)		(3.1	)
Federal and state credits		2.5			7.2			10.3
Change in valuation allowance		(27.6	)		(22.5	)		(41.5	)
Other		(0.1	)		—			—
Federal and state rate change		—			(24.6	)		—
Research and orphan drug credit addback		—			(2.4	)		(2.9	)
		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, 2018 and 2017 are as follows:

	December 31,
	2018			2017
	(in thousands)
Net operating losses	$	206,460		$	131,151
Capitalized start-up costs		1,242			1,362
Tax credit carryforwards		63,653			54,523
Accrued expenses		4,498			3,057
Depreciation and amortization		1,001			685
Stock options		30,925			13,573
Others		568			1,090
Total net deferred tax asset before valuation    allowance		308,347			205,441
Valuation allowance		(308,347	)		(205,441	)
Net deferred tax asset	$	—		$	—

Summary of Rollforward of Company's Unrecognized Tax Benefits

The following is a rollforward of the Company’s unrecognized tax benefits:

	Year Ended December 31,
	2018		2017		2016
	(in thousands)
Unrecognized tax benefits—as of the beginning of the    year	$	—	$	—	$	—
Gross increases—current period tax positions		—		—		—
Gross decreases—tax positions of prior periods		—		—		—
Unrecognized tax benefits—as of the end of the year	$	—	$	—	$	—