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Debt (Senior Credit Facility) (Details)
6 Months Ended 12 Months Ended
Jun. 30, 2020
USD ($)
Jun. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Line of Credit Facility [Line Items]      
Contributions from member $ 8,296,000 $ 6,119,000  
Term Loan | Term Loan B      
Line of Credit Facility [Line Items]      
Line of credit availability 240,700,000    
Line of credit, outstanding balance 236,400,000    
Senior credit facility      
Line of Credit Facility [Line Items]      
Remainder amount available for borrowing $ 271,000,000.0    
Senior credit facility | Maximum      
Line of Credit Facility [Line Items]      
Leverage ratio 6.50    
Secured leverage ratio 4.00    
Senior credit facility | Term Loan B      
Line of Credit Facility [Line Items]      
Basis spread on variable rate (as a percent) 2.25%    
Principal payments, a percentage of the original principal amount 0.25%    
Senior credit facility | Revolving Credit Facility      
Line of Credit Facility [Line Items]      
Line of credit availability $ 550,000,000.0    
Contributions from member     $ 50,000,000.0
Line of credit, outstanding balance $ 275,000,000.0    
Senior credit facility | Revolving Credit Facility | LIBOR | Minimum      
Line of Credit Facility [Line Items]      
Basis spread on variable rate (as a percent) 1.50%    
Senior credit facility | Revolving Credit Facility | LIBOR | Maximum      
Line of Credit Facility [Line Items]      
Basis spread on variable rate (as a percent) 2.75%    
Senior credit facility | Letters of Credit      
Line of Credit Facility [Line Items]      
Line of credit, outstanding balance $ 4,000,000.0