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Income Taxes - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Line Items]      
Other tax expense (benefit) $ 1,200,000    
Tax cuts and jobs act of 2017 reduction in deferred tax assets 4,700,000    
Tax cuts and jobs act of 2017, tax expense due to change in tax rate 4,700,000    
Valuation allowance 523,000 $ 133,000  
Uncertain tax liabilities 0 $ 0 $ 0
State      
Income Taxes [Line Items]      
Valuation allowance 297,000    
Adjustments for New Accounting Pronouncement [Member]      
Income Taxes [Line Items]      
Excess tax benefit $ 5,800,000