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Borrowings - Credit Agreement (Details) - USD ($)
1 Months Ended
Jan. 26, 2018
Jan. 31, 2016
Jun. 30, 2018
Revolving Credit      
Credit Agreement      
Current borrowing capacity   $ 150,000,000  
Credit facility initial term   364 days  
New Credit Agreement      
Credit Agreement      
Current borrowing capacity $ 150,000,000    
Credit facility initial term 364 days    
Amount outstanding under Credit Agreement     $ 0