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Financial Instruments and Derivatives - Carrying Amount and Fair Values of Financial Instruments (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Carrying Amount    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt, net of current portion, fair value $ 1,189,415 $ 1,189,717
Fair value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt, net of current portion, fair value $ 1,273,699 $ 1,270,521