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Risk Management Activities and Derivative Transactions Risk Management Activities and Derivative Transactions - Other Derivative Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2016
Sep. 30, 2015
Derivative    
Forward Contract Derivative Liability, at Fair Value   $ 59
Forward Contract Derivative Asset, at Fair Value $ 15  
Fair Value, Inputs, Level 2    
Derivative    
Forward Contract Derivative Liability, at Fair Value   $ 59
Forward Contract Derivative Asset, at Fair Value $ 15