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Other Assets (Schedule of Other Assets) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]    
Patents, at cost $ 6,814 $ 6,525
Accumulated amortization (4,202) (3,915)
Deposits on capital items 7,958 21,436
Other 3,169 3,495
Other assets $ 13,739 $ 27,541