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Fair Value Measurements Estimated Fair Values of Financial Instruments (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash, restricted cash, and short-term investments (Level 1) $ 15,738 $ 23,001
Line of Credit Facility, Fair Value of Amount Outstanding 154,882 135,000
Long-term debt (Level 1) 569,250 567,875
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash, restricted cash, and short-term investments (Level 1) 15,738 23,001
Line of Credit Facility, Fair Value of Amount Outstanding 155,000 135,000
Long-term debt (Level 1) $ 575,000 $ 575,000