Debt and Lines of Credit - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended			9 Months Ended
	May 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Commercial paper		$ 0			$ 0
Retirement of long-term debt	$ 500,000,000
Long-term debt, gross		5,100,000,000			5,100,000,000		$ 4,100,000,000
Proceeds from issuance of long-term debt					1,500,000,000	$ 605,000,000
Interest and debt expense		36,000,000		$ 19,000,000	89,000,000	$ 57,000,000
Notes due 2048 at 4.15%
Debt Instrument [Line Items]
Long-term debt, gross		1,500,000,000	$ 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
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
Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity until March 2023		$ 2,000,000,000			$ 2,000,000,000