Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Percentage of annual research and development credit for cash	65.00%
Valuation allowance on the deferred tax asset	100.00%	100.00%
Change in valuation allowance	$ 23,642,000	$ 618,000
Cumulative-effect adjustment for tax benefit on exercise of non-qualified stock options and disqualified disposition of incentive stock options		840,000
Unrecognized tax benefits, related interest and penalties	$ 0	$ 0
Tax examinations, description	Tax years 2006 through 2017 remain open to U.S. federal and state tax examinations.
Open tax years	2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
Income tax rate	21.00%	34.00%	34.00%
Reduction in deferred tax assets		$ 25,913,000
Maximum [Member]
Income Taxes [Line Items]
Income tax rate		35.00%
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 274,764,000
Net operating losses on taxable income, threshold percentage in carryforward year	80.00%
Federal [Member] | R&D Tax Credit Carryforwards [Member]
Income Taxes [Line Items]
Tax credit carryforwards	$ 9,925,000
Federal [Member] | Earliest Tax Year [Member]
Income Taxes [Line Items]
Tax loss carryforwards expiration year	2026
Federal [Member] | Earliest Tax Year [Member] | R&D Tax Credit Carryforwards [Member]
Income Taxes [Line Items]
Tax credit expiration year	2025
State and Local [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 267,973,000
State and Local [Member] | R&D Tax Credit Carryforwards [Member]
Income Taxes [Line Items]
Tax credit carryforwards	$ 1,236,000
State and Local [Member] | Earliest Tax Year [Member]
Income Taxes [Line Items]
Tax loss carryforwards expiration year	2027