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Financial Instruments Financial Instruments narrative (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
May 01, 2018
Dec. 31, 2017
Convertible Debt, Fair Value Disclosures   $ 651,300  
Debt securities gross unrealized gains $ 200    
Debt instrument fair value senior notes 2023 762,900    
Debt instrument fair value senior notes 2025 511,500    
Debt instrument fair value senior notes 2027 536,100    
debt instruments discount rate   8.42%  
Fair Value, Inputs, Level 1 [Member]      
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value $ 972,682   $ 2,169,197