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Merger-Related Activities (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Business Acquisition [Line Items]      
Merger-related $ 0us-gaap_BusinessCombinationAcquisitionRelatedCosts $ 52us-gaap_BusinessCombinationAcquisitionRelatedCosts $ 0us-gaap_BusinessCombinationAcquisitionRelatedCosts
BHE Merger [Member]      
Business Acquisition [Line Items]      
One-time bill credit   15npc_Credittoretailcustomers
/ us-gaap_BusinessAcquisitionAxis
= npc_NVEnergyTransactionMember		  
Merger-related   52us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= npc_NVEnergyTransactionMember		  
Change in control costs [Member] | BHE Merger [Member]      
Business Acquisition [Line Items]      
Merger-related   15us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= npc_NVEnergyTransactionMember
/ npc_BusinessAcquisitionCostsbyTypeAxis
= npc_ChangeinControlCostsMember		  
Stock compensation plan costs [Member] | BHE Merger [Member]      
Business Acquisition [Line Items]      
Merger-related   18us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= npc_NVEnergyTransactionMember
/ npc_BusinessAcquisitionCostsbyTypeAxis
= us-gaap_StockCompensationPlanMember		  
Investment banker fees [Member] | BHE Merger [Member]      
Business Acquisition [Line Items]      
Merger-related   15us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= npc_NVEnergyTransactionMember
/ npc_BusinessAcquisitionCostsbyTypeAxis
= npc_InvestmentBankerFeesMember		  
Legal fees and other expenses [Member] | BHE Merger [Member]      
Business Acquisition [Line Items]      
Merger-related   $ 4us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= npc_NVEnergyTransactionMember
/ npc_BusinessAcquisitionCostsbyTypeAxis
= npc_LegalFeesandOtherExpensesMember