Debt and Lines of Credit - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Apr. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Commercial paper	$ 0
Long-term debt, gross	4,100,000,000			$ 4,100,000,000
Interest and debt expense	23,000,000	$ 18,000,000
Subsequent Event
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,300,000,000
Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity until March 2023	$ 2,000,000,000