Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Summary of Financial Instruments at Carrying and Fair Value) (Details)

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Fair Value Measurements (Summary of Financial Instruments at Carrying and Fair Value) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Jun. 07, 2018
Dec. 31, 2017
May 19, 2017
Oct. 03, 2016
6.125% senior unsecured notes due 2024          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Debt instrument, interest rate, stated (as a percent) 6.125%     6.125%  
6.125% senior unsecured notes due 2024 | Unsecured debt          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Debt instrument, interest rate, stated (as a percent) 6.125%     6.125% 6.125%
6.375% senior unsecured notes due 2026          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Debt instrument, interest rate, stated (as a percent) 6.375%        
6.375% senior unsecured notes due 2026 | Unsecured debt          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Debt instrument, interest rate, stated (as a percent) 6.375% 6.375%      
Carrying Value          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Credit Facility $ 65,000   $ 25,000    
Total 1,053,528   620,196    
Carrying Value | 6.125% senior unsecured notes due 2024 | Unsecured debt          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Senior Notes 595,729   595,196    
Carrying Value | 6.375% senior unsecured notes due 2026 | Unsecured debt          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Senior Notes 392,799   0    
Fair Value          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Credit Facility 0   0    
Total 1,019,500   618,000    
Fair Value | 6.125% senior unsecured notes due 2024 | Unsecured debt          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Senior Notes 610,500   618,000    
Fair Value | 6.375% senior unsecured notes due 2026 | Unsecured debt          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Senior Notes $ 409,000   $ 0