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Property and Equipment (Narrative) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Depreciation expense $ 29,302 $ 25,561 $ 24,331
Impairment of assets 1,998 2,427 8,618
Property, Plant and Equipment      
Impairment of assets $ 625 $ 636 $ 7,408