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Financial Instruments - Narrative (Details) - Fair value inputs, Level 2 - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Carrying Value | 6% Senior notes    
Debt Instrument [Line Items]    
Financial Liabilities Fair Value Disclosure   $ 496,531
Carrying Value | Five Percent Senior Notes Due Two Thousand Twenty Eight [Member]    
Debt Instrument [Line Items]    
Financial Liabilities Fair Value Disclosure $ 491,648  
Fair Value | 6% Senior notes    
Debt Instrument [Line Items]    
Financial Liabilities Fair Value Disclosure $ 477,835 $ 522,045