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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
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial investments, at fair value $ 220 $ 245
Fair Value, Measurements, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial investments, at fair value 220 245
Default fund and margin deposit investments 2,186 1,900
Total 2,406 2,145
Fair Value, Measurements, Recurring | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial investments, at fair value 203 228
Default fund and margin deposit investments 1,610 1,763
Total 1,813 1,991
Fair Value, Measurements, Recurring | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial investments, at fair value 17 17
Default fund and margin deposit investments 576 137
Total 593 154
Fair Value, Measurements, Recurring | 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