Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
U.S. federal statutory tax rate		34.00%	34.00%	34.00%
Provisional estimate of reduction in deferred tax assets		$ 24,196,000
Provisional estimate of reduction in valuation allowance		$ 24,196,000
Period of incurred losses for both book and tax purposes		3 years
Valuation allowance for deferred tax assets		$ 52,038,000	$ 67,758,000
Increased and decreased total valuation allowance		15,720,000	8,864,000
Deferred tax assets, net of valuation allowances		0	0
Net operating loss carry forward unused amount		428,000
Unrecognized tax benefits		0	0
Interest or penalties charged in relation to unrecognized tax benefits		0	0
Interest or penalties accrued in relation to unrecognized tax benefits		$ 0	$ 0
Total unrecognized tax benefits changing period		12 months
U.S Federal Tax [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carry forward		$ 176,069,000
Research credit carryforward		5,617,000
Foreign Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carry forward		$ 454,000
Tax credit carryforward expiration year		2018
Maximum [Member]
Operating Loss Carryforwards [Line Items]
U.S. federal statutory tax rate		35.00%
Maximum [Member] | U.S Federal Tax [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward expiration year		2037
Tax credit carryforward expiration year		2037
Minimum [Member] | U.S Federal Tax [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward expiration year		2018
Tax credit carryforward expiration year		2018
Scenario, Plan [Member]
Operating Loss Carryforwards [Line Items]
U.S. federal statutory tax rate	21.00%