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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, 2014
Dec. 31, 2013
Sep. 30, 2013
Fair Value, Inputs, Level 2
     
Debt Instrument [Line Items]      
Estimated Fair Value of Long-Term Debt $ 378.2 $ 327.3 $ 326.7