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Financial Instruments Financial Instruments narrative (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument, Fair Value Disclosure $ 4,200,000  
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value 2,169,172 $ 1,626,764
Available-for-sale Debt Securities Gross Unrealized Gain 1,000  
Due 2023 [Member]    
Debt Instrument, Fair Value Disclosure 772,900  
Due 2025 [Member]    
Debt Instrument, Fair Value Disclosure 544,400  
Due 2027 [Member]    
Debt Instrument, Fair Value Disclosure $ 563,700