Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes Disclosure [Line Items]
Statutory income tax rate	21.00%	35.00%	35.00%
Valuation allowance, deferred tax asset, increase (decrease), amount		$ (248,200,000)
Valuation allowance, increase (decrease) amount	$ (35,700,000)	(169,300,000)	$ 52,500,000
Valuation allowance related to stock-based compensation included in income tax expense	(35,600,000)	(35,600,000)
Additional income taxes for remaining undistributed foreign earnings	0
Income tax research credits	17,295,000	8,038,000	4,543,000
Federal orphan drug credits	$ 17,700,000
Federal orphan drug credits, expiration year	2026
Unrecognized tax benefits, period may increase or decrease due to tax examination	Next twelve months
Unrecognized tax benefits, interest recognized	$ 0	0	0
Unrecognized tax benefits, penalties recognized	0	$ 0	$ 0
Federal [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforward for income tax	$ 1,304,100,000
Net operating loss carryforward, expiration year	2022
Income tax research credits	$ 101,000,000
Income tax research credits, expiration year	2020
State [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforward for income tax	$ 471,100,000
Net operating loss carryforward, expiration year	2024
Income tax research credits	$ 40,000,000