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Income Taxes - Reconciliation of Income Tax Expense (Parenthetical) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Valuation Allowance [Line Items]      
Computed income tax expense at statutory federal income tax rate of 35%, tax rate 35.00% 35.00% 35.00%
Increase to valuation allowance related to remeasurement of deferred taxes due to reduction in U.S. tax rate $ (796) $ 1,230 $ (944)
U.S      
Valuation Allowance [Line Items]      
Increase to valuation allowance related to remeasurement of deferred taxes due to reduction in U.S. tax rate $ 600