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Asset Retirement Obligations Asset Retirement Obligations - NPC - Change in Asset Retirement Obligations (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning Balance $ 753 $ 696
Asset Retirement Obligation, Revision of Estimate 104 3
Asset Retirement Obligation, Liabilities Incurred 59 15
Retirements (32) (8)
Asset Retirement Obligation, Accretion Expense 37 35
Ending Balance 921 753
Asset Retirement Obligation, Current 92 66
Asset Retirement Obligations, Noncurrent 829 687
Nevada Power Company [Member]    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning Balance 86 100
Asset Retirement Obligation, Revision of Estimate 3 (18)
Asset Retirement Obligation, Liabilities Incurred 3 0
Retirements (11) 0
Asset Retirement Obligation, Accretion Expense 4 4
Ending Balance 85 86
Asset Retirement Obligation, Current 13 14
Asset Retirement Obligations, Noncurrent $ 72 $ 72