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Financing Agreement - Additional Information (Detail) (USD $)
9 Months Ended1 Months Ended
Sep. 30, 2011
Aug. 31, 2011
Mar. 31, 2011
Dec. 31, 2010
Dec. 31, 2010
Master Notes
Dec. 31, 2010
Master Notes
Senior Unsecured Notes, 3.81 Percent, Due May 2018
Dec. 31, 2010
Master Notes
Senior Unsecured Notes, 4.60 Percent, Due May 2021
Line of Credit Facility [Line Items]       
Borrowings under credit agreement$ 250,000,000      
Line of credit outstanding amount0 200,000,000200,000,000   
Cash on hand100,000,000      
Loan agreement expiration date2016-08      
Long term debt100,000,000 100,000,000100,000,000   
Unsecured revolving loan facility 350,000,000     
Issuance of senior notes     $ 25,000,000$ 75,000,000
Interest rate for purchase agreement     3.81%4.60%
Maturity date     2018-052021-05
Issuance date of senior notes    2011-05