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Changes in level 3 assets measured at fair value (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Assets, Fair Value Disclosure [Abstract]    
Fair value of plan assets at end of year $ 697,240us-gaap_DefinedBenefitPlanFairValueOfPlanAssets $ 705,482us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Fair Value, Inputs, Level 3 [Member]    
Assets, Fair Value Disclosure [Abstract]    
Fair value of plan assets at beginning of year 2,562us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
2,848us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Defined Benefit Plan, Actual Return on Plan Assets Still Held (459)us-gaap_DefinedBenefitPlanActualReturnOnPlanAssetsStillHeld
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
(286)us-gaap_DefinedBenefitPlanActualReturnOnPlanAssetsStillHeld
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Defined Benefit Plan, Purchases, Sales, and Settlements 0us-gaap_DefinedBenefitPlanPurchasesSalesAndSettlements
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0us-gaap_DefinedBenefitPlanPurchasesSalesAndSettlements
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Fair value of plan assets at end of year 2,103us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
2,562us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Private Equity Investment [Member]    
Assets, Fair Value Disclosure [Abstract]    
Fair value of plan assets at end of year 546us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_PrivateEquityFundsMember
922us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_PrivateEquityFundsMember
Private Equity Investment [Member] | Fair Value, Inputs, Level 3 [Member]    
Assets, Fair Value Disclosure [Abstract]    
Fair value of plan assets at end of year $ 546us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_PrivateEquityFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
$ 922us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_PrivateEquityFundsMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member