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Subsequent Event - Additional Information (Detail) (Senior Unsecured Notes, 3.75 Percent, Due April 27th, 2018, Issuance of Debt, USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2011
Senior Unsecured Notes, 3.75 Percent, Due April 27th, 2018 | Issuance of Debt
 
Subsequent Event [Line Items]  
Senior unsecured notes, issuance date 2011-07-05
Senior unsecured notes, aggregate principal amount $ 125.0
Senior unsecured notes, interest rate 3.75%
Senior unsecured notes, maturity date Apr. 27, 2018
Senior unsecured notes, date of first required interest payment 2011-07-27
Senior unsecured notes, required principal payments $ 25.0
Senior unsecured notes, date of first required principal payments 2014-04-27