Short-Term Debt - Credit Facilities (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Mar. 24, 2015	Dec. 17, 2013
Line Of Credit Facility [Line Items]
Credit Facilities	$ 475,000,000	$ 475,000,000		$ 200,000,000
Borrowings Outstanding	170,000,000	61,000,000
Letters of Credit Outstanding	500,000	500,000
5-year Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	325,000,000	325,000,000
Borrowings Outstanding	40,000,000	0
Letters of Credit Outstanding	500,000	500,000
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	130,000,000	61,000,000
Letters of Credit Outstanding	$ 0	$ 0