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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
Sep. 30, 2018
Dec. 31, 2017
Assets at Fair Value    
Financial investments, at fair value $ 202 $ 235
Fair Value, Measurements, Recurring    
Assets at Fair Value    
Financial investments, at fair value 202 235
Default fund and margin deposit investments 1,617 2,129
Total Assets at Fair Value 1,819 2,364
Liabilities at Fair Value    
Other financial instruments 56  
Total Liabilities at Fair Value 56  
Fair Value, Measurements, Recurring | Level 1    
Assets at Fair Value    
Financial investments, at fair value 148 135
Default fund and margin deposit investments 215 371
Total Assets at Fair Value 363 506
Liabilities at Fair Value    
Other financial instruments 0  
Total Liabilities at Fair Value 0  
Fair Value, Measurements, Recurring | Level 2    
Assets at Fair Value    
Financial investments, at fair value 54 100
Default fund and margin deposit investments 1,402 1,758
Total Assets at Fair Value 1,456 1,858
Liabilities at Fair Value    
Other financial instruments 56  
Total Liabilities at Fair Value 56  
Fair Value, Measurements, Recurring | Level 3    
Assets at Fair Value    
Financial investments, at fair value 0 0
Default fund and margin deposit investments 0 0
Total Assets at Fair Value 0 $ 0
Liabilities at Fair Value    
Other financial instruments 0  
Total Liabilities at Fair Value $ 0