Short-Term Debt - Credit Facilities and Related Borrowings (Parenthetical) (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
	Mar. 23, 2018	Sep. 30, 2018	Dec. 31, 2017
Line Of Credit Facility [Line Items]
Long-term debt due within one year		$ 0	$ 304
5-year Facility [Member]
Line Of Credit Facility [Line Items]
Credit facility maturity date		Mar. 22, 2022	Mar. 22, 2022
3-year Accounts Receivable Facility [Member]
Line Of Credit Facility [Line Items]
Credit facility maturity date	Mar. 22, 2021	Mar. 22, 2021	Mar. 22, 2021
1-year Term Facility [Member]
Line Of Credit Facility [Line Items]
Credit facility maturity date		Oct. 11, 2018	Oct. 11, 2018
Long-term debt due within one year		$ 300	$ 300