Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

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,
	2017			2016			2015
Tax due at statutory rate		34.0	%		34.0	%		34.0	%
State taxes, net of federal		5.1			4.2			4.0
Stock-based compensation		6.4			(1.0	)		(0.9	)
Foreign rate differential		(3.2	)		(3.1	)		(3.1	)
Federal and state credits		7.2			10.3			13.2
Change in valuation allowance		(22.5	)		(41.5	)		(43.1	)
Other		—			—			0.1
Federal and state rate change		(24.6	)		—			—
Research and orphan drug credit addback		(2.4	)		(2.9	)		(4.2	)
		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, 2017 and 2016 are as follows:

	December 31,
	2017			2016
	(in thousands)
Net operating losses	$	131,151		$	84,374
Capitalized start-up costs		1,362			2,155
Accounting method change		—			(673	)
Tax credit carryforwards		54,523			33,477
Accrued expenses		3,057			2,287
Depreciation and amortization		685			999
Stock options		13,573			12,103
Others		1,090			350
Total net deferred tax asset before valuation    allowance		205,441			135,072
Valuation allowance		(205,441	)		(135,072	)
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,
	2017		2016		2015
	(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	$	—	$	—	$	—