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Long-Term Debt - Partnership Credit Facility (Details) - Revolving Credit Facility - Partnership Credit Facility - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Line of Credit Facility      
Letter of credit outstanding $ 15,200,000    
Debt instrument weighted average interest rate (percent) 5.30%   5.40%
Commitment fee amount $ 500,000 $ 500,000  
Undrawn capacity under revolving credit facility 834,300,000    
Maximum      
Line of Credit Facility      
Undrawn capacity under revolving credit facility $ 485,800,000    
Base rate      
Line of Credit Facility      
Debt instrument, variable rate (percentage) 2.70%