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Schedule of Target Assets Allocation (Detail)
 12 Months Ended
Dec. 31, 2014
Passive And Active Fixed Income | Barclays U.S. Long Government/Credit Bond Index | Fixed Income Funds  
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]  
Target assets allocation percentage 85.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
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Equity (Large Cap) | S&P 500 Stock Index | Equity Securities  
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]  
Target assets allocation percentage 10.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
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International Equity | MSCI EAFE Index | Equity Securities  
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]  
Target assets allocation percentage 5.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations
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