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Income taxes - Schedule of Benefit for Income Taxes (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Current      
Federal     $ (123)
State   $ 87  
Current Federal, State and Local, Tax Expense (Benefit), Total   87 (123)
Deferred      
Federal $ (7,117) (14,278) (15,771)
State (1,855) (1,732) (689)
Valuation allowance $ 8,972 1,881  
Deferred Federal, State and Local, Tax Expense (Benefit), Total   (14,129) (16,460)
Income tax benefit   $ (14,042) $ (16,583)