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Debt - Summary (Details) - USD ($) $ in Millions	1 Months Ended
Debt - Summary (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Sep. 22, 2016	Aug. 18, 2015	May 26, 2015
Debt Instrument [Line Items]
Total debt		$ 3,662	$ 3,569
Less: short-term debt including current maturities		0	399
Long-term debt		3,662	3,170
Senior Notes \| 2.5% Senior Notes, due 2018
Debt Instrument [Line Items]
Senior notes		$ 0	399
Interest rate (as a percent)		2.50%			2.50%
Early repayment of debt	$ 400
Senior Notes \| 3.3% Senior Notes, due 2020
Debt Instrument [Line Items]
Senior notes		$ 698	697
Interest rate (as a percent)		3.30%			3.30%
Unamortized debt discount and issuance costs		$ 2
Senior Notes \| 4.0% Senior Notes, due 2025
Debt Instrument [Line Items]
Senior notes		$ 693	692
Interest rate (as a percent)		4.00%				4.00%
Unamortized debt discount and issuance costs		$ 7
Senior Notes \| 4.4% Senior Notes, due 2026
Debt Instrument [Line Items]
Senior notes		$ 892	892
Interest rate (as a percent)		4.40%			4.40%
Unamortized debt discount and issuance costs		$ 8
Senior Notes \| 2.95% Senior Notes, due 2027
Debt Instrument [Line Items]
Senior notes		$ 493	493
Interest rate (as a percent)		2.95%		2.95%
Unamortized debt discount and issuance costs		$ 7
Senior Notes \| 6.55% Senior Notes, due 2037
Debt Instrument [Line Items]
Senior notes		$ 396	396
Interest rate (as a percent)		6.55%
Unamortized debt discount and issuance costs		$ 4
Senior Notes \| 4.5% Senior Notes, due 2048
Debt Instrument [Line Items]
Senior notes		$ 490	$ 0
Interest rate (as a percent)		4.50%
Unamortized debt discount and issuance costs		$ 10