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Commitments and Contingencies - State and Local Taxes and Surcharges (Details)
 3 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Sep. 30, 2016
USD ($)
Jun. 30, 2016
USD ($)
Mar. 31, 2016
USD ($)
Mar. 31, 2015
USD ($)
state
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Loss Contingencies [Line Items]                        
General and administrative expense $ 8,767,000 $ 4,679,000 $ 6,845,000 $ 8,860,000 $ 6,511,000 $ 6,143,000 $ 5,882,000 $ 6,536,000   $ 29,151,000 $ 25,072,000 $ 25,822,000
Sales tax liability - current 1,326,000       1,347,000         1,326,000 1,347,000  
Contingent sales tax liabilities                        
Loss Contingencies [Line Items]                        
Number of states with sales tax liability | state                 4      
General and administrative expense                 $ 600,000      
Payments on usage-based fees                   0 300,000 1,100,000
Sales tax liability expense (income)                   (600,000) (400,000) $ 1,200,000
Sales tax liability 1,500,000       2,100,000         1,500,000 2,100,000  
Sales tax liability - current $ 400,000       $ 600,000         $ 400,000 $ 600,000