Subsequent Event - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Oct. 22, 2012
Subsequent Event [Line Items]
Interest Payment terms	Interest will be payable in arrears semi-annually on June 12, and December 12, beginning on June 12, 2013.
Note Purchase Agreement
Subsequent Event [Line Items]
Number of Institutional Purchaser	46
Series A notes
Subsequent Event [Line Items]
Unsecured senior notes to be issued, face value	125.0
Unsecured senior notes to be issued, interest rate	2.68%
Unsecured senior notes to be issued, interest rate	7 years
Unsecured senior notes to be issued, maturity date	Dec. 12, 2019
Series B notes
Subsequent Event [Line Items]
Unsecured senior notes to be issued, face value	125.0
Unsecured senior notes to be issued, interest rate	3.26%
Unsecured senior notes to be issued, interest rate	10 years
Unsecured senior notes to be issued, maturity date	Dec. 12, 2022