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Estimated Fair Value of Long-Term Debt (Detail) (Fair Value, Inputs, Level 2, USD $)
In Millions, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Sep. 30, 2012
Fair Value, Inputs, Level 2
     
Debt Instrument [Line Items]      
Estimated Fair Value of Long-Term Debt $ 326.7 $ 349.7 $ 343.6