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Fair Value of Financial Instruments (Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial investments, at fair value $ 245 $ 201
Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial investments, at fair value 245 201
Default fund and margin deposit investments 1,900 1,556
Total 2,145 1,757
Fair Value, Measurements, Recurring [Member] | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial investments, at fair value 228 189
Default fund and margin deposit investments 1,763 1,253
Total 1,991 1,442
Fair Value, Measurements, Recurring [Member] | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial investments, at fair value 17 12
Default fund and margin deposit investments 137 303
Total 154 315
Fair Value, Measurements, Recurring [Member] | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial investments, at fair value 0 0
Default fund and margin deposit investments 0 0
Total $ 0 $ 0