Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Effective tax rate		1.00%	0.00%	0.00%
Change in valuation allowance		$ 10,529
Tax rate		34.00%	34.00%	34.00%
Deferred tax assets, valuation allowance		$ 32,325	$ 24,888
Change in valuation allowance		7,437
Federal net operating loss carry-forwards		91,051
State net operating loss carry-forwards		$ 112,876
Minimum maturity period for federal net operating loss carry-forward		5 years
Maximum maturity period for federal net operating loss carry-forward		20 years
Minimum maturity period for state net operating loss carry-forward		5 years
Maximum maturity period for state net operating loss carry-forward		20 years
Federal research and experimental tax credits		$ 5,103
State research and experimental tax credits		$ 1,899
Minimum maturity period for federal research and experimental tax credit carry-forward		5 years
Maximum maturity period for federal research and experimental tax credit carry-forward		20 years
Minimum maturity period for state research and experimental tax credit carry-forward		5 years
Maximum maturity period for state research and experimental tax credit carry-forward		20 years
Accrued interest and penalties associated with uncertain tax positions		$ 0	$ 0	$ 4
Scenario Forecast [Member]
Income Tax Disclosure [Line Items]
Tax rate	21.00%