Debt and Lines of Credit - Additional Information (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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		30,000,000	$ 20,000,000	53,000,000	$ 38,000,000
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
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		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