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Fair Value Measurements - Fair Value of Debt (Details) - Fixed rate debt - Level 2 - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Long-term debt, fair value $ 836,314 $ 690,001
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Long-term debt, fair value $ 881,000 $ 674,000