Short-Term Debt - Credit Facilities and Related Borrowings (Detail) - USD ($)	Sep. 30, 2020	Jul. 14, 2020	Dec. 31, 2019
Line Of Credit Facility [Line Items]
Credit Facilities	$ 850,000,000		$ 550,000,000
Borrowings Outstanding	535,000,000		348,000,000
Letters of Credit Outstanding	1,000,000		1,000,000
5-year Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	400,000,000		400,000,000
Borrowings Outstanding	195,000,000		295,000,000
Letters of Credit Outstanding	1,000,000		1,000,000
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
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	40,000,000		53,000,000
Letters of Credit Outstanding	$ 0		$ 0