Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes Disclosure [Line Items]
Statutory income tax rate	35.00%
Increase in valuation allowance	$ 52,500	$ 17,300
Valuation allowance related to stock-based compensation and exercises prior to the implementation of ASC 515 and 718	35,600	35,600
Deferred U.S. income taxes	0	0	$ 0
Income taxes have not been provided on a cumulative total	3,400
Income tax research credits	4,543	$ 4,529	$ 2,933
Federal orphan drug credits	$ 17,700
Federal orphan drug credits, expiration year	2026
Unrecognized tax benefits, period may increase or decrease due to tax examination	Next twelve months
Federal [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforward for income tax	$ 1,413,300
Net operating loss carryforward, expiration year	2018
Income tax research credits	$ 50,100
Income tax research credits, expiration year	2019
State [Member]
Income Taxes Disclosure [Line Items]
Net operating loss carryforward for income tax	$ 473,600
Net operating loss carryforward, expiration year	2017
Income tax research credits	$ 25,200