Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	$ 2	$ 29
Net derivative contracts	(66)	41
Available for sale investments	22	16
Held to maturity investments	60	0
Quoted Prices In Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	2	29
Net derivative contracts	0	0
Available for sale investments	22	16
Held to maturity investments	60	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Net derivative contracts	(66)	41
Available for sale investments	0	0
Held to maturity investments	0	0
Total Cost Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	2	29
Net derivative contracts	0	0
Available for sale investments	6	4
Held to maturity investments	$ 60	$ 0