Short-Term Debt - Credit Facilities and Related Borrowings (Detail) - USD ($)	Sep. 30, 2017	Mar. 22, 2017	Dec. 31, 2016
Line Of Credit Facility [Line Items]
Credit Facilities	$ 475,000,000	$ 50,000,000	$ 475,000,000
Borrowings Outstanding	255,000,000		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	170,000,000		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
Borrowings Outstanding	85,000,000		130,000,000
Letters of Credit Outstanding	$ 0		$ 0