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Supplemental Consolidated Financial Statement Information - Disaggregated Revenues, Deferred Costs and Deferred Revenue (Details)
 3 Months Ended 12 Months Ended
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2018
USD ($)
segment
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Number of business segments | segment                 1    
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
Deferred cost         2,991,000         2,991,000 3,668,000
Deferred costs incurred to fulfill contracts with customers 6,994,000               6,994,000    
Total deferred costs amortized                 36,615,000 39,980,000  
Deferred revenue $ 10,501,000       $ 6,314,000       10,501,000 6,314,000 5,385,000
Revenue recognized that was included in deferred revenue balances at beginning of period                 5,945,000 5,381,000  
United States                      
Operating revenues                 137,101,000 134,442,000 126,832,000
Canada                      
Operating revenues                 $ 17,055,000 $ 22,090,000 $ 10,808,000