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Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]    
Gas Cash Flow hedges $ 0 $ 0
Guarantees, Fair Value Disclosure (1,285) (1,362)
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]    
Gas Cash Flow hedges (163,515) (188,156)
Guarantees, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]    
Gas Cash Flow hedges 0 0
Guarantees, Fair Value Disclosure $ 0 $ 0