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Fair Value Measurements (Fair Value Hierarchy) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities $ 64 $ 987
Derivative Assets 253 170
Derivative liabilities 78 354
Recurring fair value measurements | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 64 987
Derivative Assets 0 0
Derivative liabilities 0 0
Recurring fair value measurements | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 0 0
Derivative Assets 253 170
Derivative liabilities 78 354
Recurring fair value measurements | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 0 0
Derivative Assets 0 0
Derivative liabilities $ 0 $ 0