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Debt and Lines of Credit - Additional Information (Details) - USD ($)	1 Months Ended						3 Months Ended	12 Months Ended
	May 31, 2018	Nov. 30, 2017	Jun. 30, 2017	May 31, 2017	Mar. 31, 2017	May 31, 2016	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Commercial paper								$ 0
Retirement of long-term debt	$ 500,000,000		$ 375,000,000		$ 250,000,000
Long-term debt, gross								5,100,000,000	$ 4,100,000,000
Proceeds from issuance of long-term debt								1,500,000,000	1,099,000,000	$ 499,000,000
Interest and debt expense								125,000,000	78,000,000	80,000,000
Interest paid								114,000,000	75,000,000	$ 88,000,000
Notes payable
Debt Instrument [Line Items]
Retirement of long-term debt						$ 1,000,000,000
Long-term debt, gross				$ 600,000,000
Payments of debt issuance costs				3,000,000
Proceeds from issuance of long-term debt				605,000,000
Notes due 2048 at 4.15%
Debt Instrument [Line Items]
Long-term debt, gross							$ 1,500,000,000	$ 1,500,000,000	$ 0
Payments of debt issuance costs							16,000,000
Proceeds from issuance of long-term debt							1,500,000,000
Long-term debt stated interest rate (in percentage)								4.15%	4.15%
4.15% Notes issued in May 2018
Debt Instrument [Line Items]
Long-term debt, gross	$ 1,300,000,000
4.15% Notes issued in June 2018
Debt Instrument [Line Items]
Long-term debt, gross							$ 200,000,000
Notes due 2021 at 2.75%
Debt Instrument [Line Items]
Long-term debt, gross				$ 300,000,000				$ 550,000,000	$ 550,000,000
Long-term debt stated interest rate (in percentage)				2.75%				2.75%	2.75%
Notes due 2024 at 2.625%
Debt Instrument [Line Items]
Long-term debt, gross				$ 300,000,000				$ 300,000,000	$ 300,000,000
Long-term debt stated interest rate (in percentage)				2.625%				2.625%	2.625%
Notes due 2027 at 2.90%
Debt Instrument [Line Items]
Long-term debt, gross		$ 500,000,000						$ 500,000,000	$ 500,000,000
Payments of debt issuance costs		3,000,000
Proceeds from issuance of long-term debt		$ 494,000,000
Long-term debt stated interest rate (in percentage)								2.90%	2.90%
Notes due 2022 at 1.85%
Debt Instrument [Line Items]
Long-term debt, gross						500,000,000		$ 500,000,000	$ 500,000,000
Payments of debt issuance costs						3,000,000
Proceeds from issuance of long-term debt						$ 499,000,000
Long-term debt stated interest rate (in percentage)								1.85%	1.85%
Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity until March 2023								$ 2,000,000,000