Income Taxes - Summary of Estimated Loss and Tax Credit Carryforwards (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Tax Credit Carryforward [Line Items]
Federal net operating loss carryforward	$ 10,586
State net operating loss carryforwards	3,364
Federal
Tax Credit Carryforward [Line Items]
Federal general business credits	2,486
Capital loss carryforwards	2,153
State
Tax Credit Carryforward [Line Items]
Capital loss carryforwards	$ 599