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Estimated Fair Value of Long Term Debt (Detail) (Fair Value, Inputs, Level 2, USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Inputs, Level 2
   
Debt Instrument [Line Items]    
Estimated Fair Value of Long-Term Debt $ 380.6us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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$ 327.3us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member