Short-Term Debt - Credit Facilities (Detail) - USD ($)	Dec. 31, 2017	Mar. 22, 2017	Dec. 31, 2016	Mar. 24, 2015
Line Of Credit Facility [Line Items]
Credit Facilities	$ 775,000,000	$ 50,000,000	$ 475,000,000
Borrowings Outstanding	305,000,000		170,000,000
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	5,000,000		40,000,000
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	0		130,000,000
Letters of Credit Outstanding	0		0
1-year Term Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	300,000,000		0
Borrowings Outstanding	300,000,000		0
Letters of Credit Outstanding	$ 0		$ 0