Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Accrual on interest and penalties related to income taxes		$ 0
Uncertain tax position	$ 2,000,000	2,000,000	$ 2,000,000
Reduction in deferred tax assets upon enactment of Tax Cuts and Jobs Act	$ 7,900,000
Removal of net operating losses from deferred tax assets		$ 17,300,000
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		$ 3,200,000
Percentage of taxable income offset by tax losses		80.00%
Changes to unrecognized tax benefits		$ 0	$ 0
Unrecognized tax benefits that would affect company's effective tax rate, if recognized		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]
Uncertain tax position		$ 1,900,000
Net operating loss carry forwards		$ 41,700,000
Expiry date of carry forwards		Dec. 31, 2028
Tax credit carry forwards		$ 1,300,000
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards		$ 68,300,000
Expiry date of carry forwards		Dec. 31, 2027
Tax credit carry forwards		$ 2,200,000