Debt and Credit Facilities (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Aug. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Apr. 30, 2018	Dec. 31, 2017
Proceeds from issuance of debt		$ 600
Commercial Paper		0
Other Long-term Debt		28			$ 35
Total debt		7,203			7,222
Less current maturities of debt		1,275			632
Total long-term debt		5,928			6,590
Unamortized debt issuance costs		39			39
Available borrowing capacity		1,553			1,547
Fair value of senior unsecured notes		$ 7,118			7,521
Senior Unsecured Notes 1.800% due 2019
Stated interest rate		1.80%	3.65%
Unsecured Debt		$ 500			499
Maturity Date		Jan. 15, 2019
Senior Unsecured Notes 1.30% due 2018
Stated interest rate				1.30%
Unsecured Debt		$ 0		$ 600
Maturity Date		Apr. 01, 2018
Commercial Paper
Debt instrument, maturity	2 years	397 days
Maximum borrowing capacity		$ 1,500
Commercial Paper | Maximum
Commercial paper		$ 1,500
Senior Unsecured Notes 2.000% due 2019
Stated interest rate		2.00%
Unsecured Debt		$ 749			748
Maturity Date		Mar. 08, 2019
Senior Unsecured Notes 4.375% Due 2020
Stated interest rate		4.38%
Unsecured Debt		$ 499			498
Maturity Date		Jan. 15, 2020
Senior Unsecured Notes 2.625% due 2021
Stated interest rate		2.63%
Unsecured Debt		$ 747			746
Maturity Date		Mar. 15, 2021
Senior Unsecured Notes 3.375% due 2024
Stated interest rate		3.38%
Unsecured Debt		$ 584			598
Maturity Date		May 15, 2024
Senior Unsecured Notes 3.375% due 2025
Stated interest rate		3.38%
Unsecured Debt		$ 746			745
Maturity Date		Nov. 01, 2025
Senior Unsecured Notes 3.50% due 2026
Stated interest rate		3.50%
Unsecured Debt		$ 989			988
Maturity Date		Mar. 15, 2026
Senior Unsecured Notes 3.650% due 2028
Stated interest rate		3.65%
Unsecured Debt		$ 595			0
Maturity Date		Mar. 07, 2028
Senior Unsecured Notes 4.100% due 2043
Stated interest rate		4.10%
Unsecured Debt		$ 391			391
Maturity Date		Apr. 01, 2043
Senior Unsecured Notes 4.375% due 2044
Stated interest rate		4.38%
Unsecured Debt		$ 395			394
Maturity Date		May 15, 2044
Senior Unsecured Notes 4.625% due 2046
Stated interest rate		4.63%
Unsecured Debt		$ 980			$ 980
Maturity Date		Mar. 15, 2046