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Debt (Details) - USD ($)
$ in Thousands
Aug. 01, 2018
Jun. 30, 2018
Mar. 05, 2018
Dec. 31, 2017
Dec. 09, 2016
Apr. 14, 2015
Revolving Credit Agreement (Textual) [Abstract]            
Senior unsecured notes   $ 574,164   $ 574,073    
Third Amended Credit Agreement            
Revolving Credit Agreement (Textual) [Abstract]            
Amended maximum borrowing capacity   500,000        
Senior Notes | Senior Notes Due 2023            
Revolving Credit Agreement [Line Items]            
Long-term Debt   $ 575,000        
Revolving Credit Agreement (Textual) [Abstract]            
Interest rate   6.875%     6.875%  
Senior unsecured notes         $ 175,000 $ 400,000
Subsequent Event | Senior Notes | Senior Notes Due 2023            
Revolving Credit Agreement [Line Items]            
Long-term Debt $ 575,000          
Revolving Credit Agreement (Textual) [Abstract]            
Interest rate 6.875%          
Revolving Credit Facility            
Revolving Credit Agreement (Textual) [Abstract]            
Maximum borrowing capacity   $ 400,000 $ 725,000      
Revolving Credit Facility | Third Amended Credit Agreement            
Revolving Credit Agreement (Textual) [Abstract]            
Borrowings under Credit Agreement   0        
Revolving Credit Facility | Subsequent Event | Third Amended Credit Agreement            
Revolving Credit Agreement (Textual) [Abstract]            
Borrowings under Credit Agreement $ 0          
Letter of Credit            
Revolving Credit Agreement (Textual) [Abstract]            
Outstanding letters of credit   $ 3,000        
Letter of Credit | Subsequent Event            
Revolving Credit Agreement (Textual) [Abstract]            
Outstanding letters of credit $ 3,000