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Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Income Tax Disclosure [Abstract]          
Provision (benefit) for income taxes $ 3 $ 255 $ 6 $ 54  
Income tax expense     $ 200    
Provisional decrease in deferred tax assets         $ 38,900
Provisional decrease in valuation allowance         41,400
Income tax benefit due to reduction in tax rate         2,500
Increase (decrease) in valuation allowance due to deferred tax assets         $ (2,900)