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Derivative and Financial Instruments - Credit Risk (Details Textual) - USD ($)
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Aggregate fair value of all derivative instruments in a liability position $ 44,000,000 $ 55,000,000
Collateral was posted for derivative instruments in a liability position 0 $ 0
Additional collateral, either in the form of cash or letters of credit $ 44,000,000  
Trade receivables [Member] | Maximum [Member]    
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]    
Terms of financial instruments and trade receivables 30 days