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Derivative Financial Instruments - Embedded Derivatives in Commodity Contracts (Details) - Embedded derivatives in commodity contracts
$ in Millions
6 Months Ended
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Natural Gas [Member]    
Derivative [Line Items]    
Notional amount for embedded derivative in commodity contract (in Dth per day) 9,000  
Number of Renewals 2  
Embedded Derivative Renewal Term 5 years  
Embedded Derivative Fair Value of Embedded Derivative Liability Including Inception Value Allocable to Host Contract $ 43 $ 54
Electricity [Member]    
Derivative [Line Items]    
Embedded Derivative, Fair Value of Embedded Derivative Liability $ 1