Short-Term Debt - Credit Facilities and Related Borrowings (Detail) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Line Of Credit Facility [Line Items]
Credit Facilities	$ 475,000,000	$ 475,000,000
Borrowings Outstanding	136,000,000	221,000,000
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	71,000,000	131,000,000
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
Borrowings Outstanding	65,000,000	90,000,000
Letters of Credit Outstanding	$ 0	$ 0