Derivative Instruments (Forward Currency Exchange Contracts) (Details) - Foreign Exchange Contract - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount	$ 5,439	$ 5,871
Maximum term in days	586 days	548 days
Derivative, Fair Value, Net	$ 22	$ (21)
Other current assets
Derivative [Line Items]
Derivative assets	38	15
Other noncurrent assets
Derivative [Line Items]
Derivative assets	2	1
Other current liabilities
Derivative [Line Items]
Derivative liabilities	(18)	(36)
Other noncurrent liabilities
Derivative [Line Items]
Derivative liabilities		(1)
Designated
Derivative [Line Items]
Notional Amount	828	1,104
Derivative, Fair Value, Net	14	4
Designated | Other current assets
Derivative [Line Items]
Derivative assets	16	11
Designated | Other noncurrent assets
Derivative [Line Items]
Derivative assets	1	1
Designated | Other current liabilities
Derivative [Line Items]
Derivative liabilities	(3)	(7)
Designated | Other noncurrent liabilities
Derivative [Line Items]
Derivative liabilities		(1)
Non-Designated
Derivative [Line Items]
Notional Amount	4,611	4,767
Derivative, Fair Value, Net	8	(25)
Non-Designated | Other current assets
Derivative [Line Items]
Derivative assets	22	4
Non-Designated | Other noncurrent assets
Derivative [Line Items]
Derivative assets	1	0
Non-Designated | Other current liabilities
Derivative [Line Items]
Derivative liabilities	$ (15)	(29)
Non-Designated | Other noncurrent liabilities
Derivative [Line Items]
Derivative liabilities		$ 0