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Fair Value Measurements - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Estimated fair value of long term debt and Preferred Stock $ 1,245.9 $ 1,240.6
Carrying value of debt, including current portion of debt, and preferred stock   $ 1,221.8