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Areas of Operation (Details) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Areas of Operation                      
Operating revenues $ 27,670,000 $ 40,448,000 $ 36,158,000 $ 49,880,000 $ 37,418,000 $ 45,108,000 $ 31,640,000 $ 42,366,000 $ 154,156,000 $ 156,532,000 $ 137,640,000
Long-lived assets 71,541,000       86,573,000       71,541,000 86,573,000  
United States                      
Areas of Operation                      
Operating revenues                 137,101,000 134,442,000 126,832,000
Long-lived assets 62,033,000       76,751,000       62,033,000 76,751,000  
Canada                      
Areas of Operation                      
Operating revenues                 17,055,000 22,090,000 $ 10,808,000
Long-lived assets $ 9,508,000       $ 9,822,000       $ 9,508,000 $ 9,822,000