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Long-term Debt - Revolving Line of Credit (Details) - USD ($)
6 Months Ended
Jun. 30, 2018
Jul. 31, 2017
Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Line of credit facility, maximum borrowing capacity   $ 500,000,000.0
Line of credit facility, additional maximum borrowing capacity   $ 250,000,000
Commitment fee percentage 0.10%  
London Interbank Offered Rate (LIBOR) | Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 0.75%  
Federal Funds Rate | Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 0.50%  
One-Month LIBOR Rate    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 0.00%  
One-Month LIBOR Rate | Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Basis spread on variable rate 1.00%