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Fair Values of Financial Instruments - Additional Information (Detail) (Series A, Senior Notes, 4.97 Percent, Due October 5th 2020)
Dec. 31, 2011
Dec. 31, 2010
Oct. 05, 2010
Series A, Senior Notes, 4.97 Percent, Due October 5th 2020
     
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Debt interest rate 4.97% 4.97% 4.97%