Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Federal statutory income tax rate	21.00%	34.00%	34.00%
Increase (decrease) in deferred tax assets valuation allowance	$ 45,300,000	$ (28,900,000)	$ 23,800,000
Deferred tax assets, net operating loss carryforwards for federal	535,000,000
Deferred tax assets, net operating loss carryforwards for state	161,000,000
Federal research tax credit carryforwards	12,200,000
State research tax credit carryforwards	$ 13,000,000
Tax credit carryforward, expiration year	2018
Accrued interest and/or penalties	$ 0
French [Member]
Income Taxes [Line Items]
Deferred tax assets, net operating loss carryforwards for foreign	$ 130,000,000
State [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year	2018
Federal [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration year	2017
Maximum [Member]
Income Taxes [Line Items]
Federal statutory income tax rate	21.00%	35.00%