Derivative Instruments (Forward Currency Exchange Contracts) (Details) - Foreign Exchange Contract - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount	$ 5,373	$ 6,336
Maximum term in days	586 days	586 days
Derivative, Fair Value, Net	$ 106	$ 57
Other current assets
Derivative [Line Items]
Derivative assets	108	43
Other noncurrent assets
Derivative [Line Items]
Derivative assets	16	34
Other current liabilities
Derivative [Line Items]
Derivative liabilities	(17)	(20)
Other noncurrent liabilities
Derivative [Line Items]
Derivative liabilities	(1)	0
Designated
Derivative [Line Items]
Notional Amount	844	870
Derivative, Fair Value, Net	(5)	11
Designated | Other current assets
Derivative [Line Items]
Derivative assets	8	15
Designated | Other noncurrent assets
Derivative [Line Items]
Derivative assets	0	1
Designated | Other current liabilities
Derivative [Line Items]
Derivative liabilities	(12)	(5)
Designated | Other noncurrent liabilities
Derivative [Line Items]
Derivative liabilities	(1)	0
Non-Designated
Derivative [Line Items]
Notional Amount	4,529	5,466
Derivative, Fair Value, Net	111	46
Non-Designated | Other current assets
Derivative [Line Items]
Derivative assets	100	28
Non-Designated | Other noncurrent assets
Derivative [Line Items]
Derivative assets	16	33
Non-Designated | Other current liabilities
Derivative [Line Items]
Derivative liabilities	(5)	(15)
Non-Designated | Other noncurrent liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0