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Fair Value Measurements - Fair Value of Other Financial Instruments (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Fair value disclosures    
Long-term debt $ 1,219,338 $ 1,226,104
Carrying Amount    
Fair value disclosures    
Long-term debt 1,219,338 1,226,104
Estimated Fair Value    
Fair value disclosures    
Long-term debt $ 1,216,612 $ 1,202,994