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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds $ 302 $ 255
Net derivative contracts (133) (66)
Available for sale investments 21 22
Held-to-maturity investments 0 60
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 3 2
Net derivative contracts 0 0
Available for sale investments 21 22
Held-to-maturity investments 0 0
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 299 253
Net derivative contracts (133) (66)
Available for sale investments 0 0
Held-to-maturity investments 0 60
Total Cost Basis    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 302 255
Net derivative contracts 0 0
Available for sale investments 7 6
Held-to-maturity investments $ 0 $ 60