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Equity Method Investments (Narrative) (Details) - USD ($)
1 Months Ended 12 Months Ended
Nov. 03, 2016
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Nov. 02, 2016
Schedule of Equity Method Investments [Line Items]            
Cash distributions $ 1,800,000   $ 175,000 $ 3,977,000    
Asset impairment       16,560,000    
Savoy [Member]            
Schedule of Equity Method Investments [Line Items]            
Equity method investment ownership percentage 30.60%   30.60%     40.80%
Cash distributions       3,600,000    
Sunrise Energy [Member]            
Schedule of Equity Method Investments [Line Items]            
Equity method investment ownership percentage     50.00%      
Cash distributions     $ 175,000 375,000    
Savoy [Member]            
Schedule of Equity Method Investments [Line Items]            
Cash recieved by equity investment for sale of equity $ 9,500,000          
Cash distributions $ 4,400,000          
Asset impairment     $ 1,000,000 $ 2,000,000 $ 2,600,000  
Lubar Equity Fund, LLC [Member] | Savoy [Member]            
Schedule of Equity Method Investments [Line Items]            
Equity method investment ownership percentage 25.00%          
Scenario, Forecast [Member] | Savoy [Member]            
Schedule of Equity Method Investments [Line Items]            
Proceeds from the sale of investment   $ 8,000,000        
Proceeds from the sale of investment, net commissions paid   $ 7,500,000