Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes Disclosure [Line Items]
Statutory income tax rate		21.00%	21.00%
Valuation allowance, increase (decrease) amount	$ 114,600,000	$ (35,700,000)	$ (169,300,000)
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	10,511,000	17,295,000	8,038,000
Federal orphan drug credits	$ 17,700,000
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
Income Taxes Disclosure [Line Items]
Net operating loss carryforward for income tax	1,721,700,000
Income tax research credits	94,800,000
State
Income Taxes Disclosure [Line Items]
Net operating loss carryforward for income tax	1,221,300,000
Income tax research credits	$ 49,400,000