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Inventories (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Inventories [Abstract]      
Excess of current costs over carrying value of inventories $ 19,947 $ 19,177  
Positive (negative) effect of LIFO valuation method as compared to the FIFO method $ (770) $ (1,231) $ 1,448