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Areas of Operation (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Areas of Operation      
Revenues $ 157,148,000 $ 133,330,000 $ 234,685,000
Long-lived assets 86,573,000 110,917,000  
U.S.      
Areas of Operation      
Revenues 135,058,000 122,522,000 222,154,000
Long-lived assets 76,751,000 105,059,000  
Canada      
Areas of Operation      
Revenues 22,090,000 10,808,000 $ 12,531,000
Long-lived assets $ 9,822,000 $ 5,858,000