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Debt (Details) - USD ($)
May 02, 2018
Mar. 31, 2018
Dec. 31, 2017
Apr. 28, 2017
Dec. 09, 2016
Apr. 14, 2015
Revolving Credit Agreement (Textual) [Abstract]            
Senior unsecured notes   $ 574,118,000 $ 574,073,000      
Third Amended Credit Agreement            
Revolving Credit Agreement (Textual) [Abstract]            
Amended maximum borrowing capacity   500,000,000        
Senior Notes | Senior Notes Due 2023            
Revolving Credit Agreement [Line Items]            
Long-term Debt   $ 575,000,000        
Revolving Credit Agreement (Textual) [Abstract]            
Interest rate   6.875%     6.875% 6.875%
Senior unsecured notes         $ 175,000,000 $ 400,000,000
Subsequent Event | Senior Notes | Senior Notes Due 2023            
Revolving Credit Agreement [Line Items]            
Long-term Debt $ 575,000,000          
Revolving Credit Agreement (Textual) [Abstract]            
Interest rate 6.875%          
Revolving Credit Facility            
Revolving Credit Agreement (Textual) [Abstract]            
Maximum borrowing capacity   $ 400,000,000   $ 725,000,000    
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 $ 45,000,000          
Letter of Credit            
Revolving Credit Agreement (Textual) [Abstract]            
Outstanding letters of credit   $ 2,100,000        
Letter of Credit | Subsequent Event            
Revolving Credit Agreement (Textual) [Abstract]            
Outstanding letters of credit $ 3,000,000