Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Accrual on interest and penalties related to income taxes	$ 0	$ 0
Unrecognized tax benefits	1,961,595	1,961,595	$ 1,961,595
Reduction in income tax expense upon enactment of Tax Cuts and Jobs Act	7,900,000
Removal of net operating losses from deferred tax assets	15,900,000	$ 15,900,000
Cumulative change in ownership for limitation of use of net operating loss and research and development credit carryforwards		50.00%
Period of cumulative change of ownership		3 years
Removal of research and development credit from deferred tax assets	3,000,000	$ 3,000,000
Percentage of taxable income offset by tax losses		80.00%
Unrecognized tax benefits that would affect company's effective tax rate, if recognized	0	$ 0
Unrecognized tax benefits, period increase (decrease)		0
Unrecognized tax benefits, interest and penalties expense		$ 0
Earliest Tax Year [Member]
Operating Loss Carryforwards [Line Items]
Tax years subject to examination by taxing authorities		2007
California [Member]
Operating Loss Carryforwards [Line Items]
Unrecognized tax benefits	1,900,000	$ 1,900,000
Net operating loss carry forwards	41,700,000	41,700,000
Expiry date of carry forwards			Dec. 31, 2017
Tax credit carry forwards	1,200,000	1,200,000
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards	61,900,000	$ 61,900,000
Expiry date of carry forwards		Dec. 31, 2027
Tax credit carry forwards	$ 2,000,000	$ 2,000,000