Short-Term Debt - Credit Facilities and Related Borrowings (Detail) - USD ($)	Sep. 30, 2018	Mar. 23, 2018	Dec. 31, 2017
Line Of Credit Facility [Line Items]
Credit Facilities	$ 475,000,000		$ 775,000,000
Borrowings Outstanding	111,000,000		305,000,000
Letters of Credit Outstanding	1,000,000		1,000,000
5-year Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	325,000,000		325,000,000
Borrowings Outstanding	45,000,000		5,000,000
Letters of Credit Outstanding	1,000,000		1,000,000
3-year Accounts Receivable Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	150,000,000	$ 150,000,000	150,000,000
Borrowings Outstanding	66,000,000		0
Letters of Credit Outstanding	0		0
1-year Term Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	0		300,000,000
Borrowings Outstanding	0		300,000,000
Letters of Credit Outstanding	$ 0		$ 0