Annual report pursuant to Section 13 and 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
Dec. 31, 2017
Sep. 30, 2017
May 19, 2017
Dec. 31, 2016
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%   6.125%
Carrying Value          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Credit facility $ 25,000     $ 0  
Notes Payable, Fair Value Disclosure 595,196     390,219  
Total 620,196     390,219  
Fair Value          
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]          
Credit facility 0     0  
Notes Payable, Fair Value Disclosure 618,000     412,000  
Total $ 618,000     $ 412,000