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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Collateral posted $ 0  
Line of credit facility, amount outstanding 164,900 $ 160,800
6.625% senior subordinated notes due 2021 641,200 640,100
Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Estimated fair value of long-term debt $ 627,900 $ 649,900