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Summary of Significant Accounting Policies - Internal Use Software, RR & IT (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Internal-Use Software      
Recognized impairment of internal use software $ 0 $ 0 $ 0
Revenue Recognition      
Allowance for doubtful accounts (in dollars) 417 1,094  
Asset Retirement and Environmental Remediation Obligations      
Asset retirement and environmental remediation obligations $ 1,600 $ 1,500  
Income taxes      
Minimum percentage of distribution of taxable income to qualify as REIT 90.00%    
Percentage of distribution of taxable income as per company policy 100.00%    
Provision for federal income taxes $ 0    
Liabilities for uncertain positions $ 0