Short-Term Debt - Additional Information (Detail) (USD $)			9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 May 2012 [Member]	Sep. 30, 2013 March 2017 [Member]
Credit Facilities [Line Items]
Short-term debt expiration date			May 01, 2012	Mar. 01, 2017
Working capital purposes	$ 150,000,000
Short-term borrowings outstanding on the credit facility	33,000,000	0
Maximum borrowing capacity	300,000,000		300,000,000
Current borrowing capacity				$ 300,000,000
Weighted-average interest rate	1.62%