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Debt - Senior Secured and Senior Unsecured Credit Facility (Details) - USD ($)
$ in Thousands
May 18, 2017
Mar. 31, 2018
Dec. 31, 2017
Debt      
Borrowings outstanding   $ 480,000 $ 480,000
Senior Unsecured Credit Facility      
Debt      
Maximum borrowing capacity $ 155,000 155,000  
Conditional maximum borrowing capacity $ 250,000    
Letter of credit borrowing percentage 50.00%    
Borrowings outstanding   $ 0 $ 0
Commitment fee (percent) 0.50%    
Senior Unsecured Credit Facility | LIBOR      
Debt      
Variable rate (percent) 3.25%    
Senior Unsecured Credit Facility | Prime Rate      
Debt      
Variable rate (percent) 2.25%