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Income Taxes Income Taxes - NPC (Details)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Federal statutory income tax rate 21.00% 35.00% 21.00% 35.00% 35.00%
Effective Income Tax Rate Reconciliation Regulatory Differences 2.00% 0.00% 3.00% 0.00%  
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent (1.00%) (1.00%) (1.00%) 0.00%  
Effective income tax rate (2.00%) (15.00%) 19.00% (13.00%)  
Tax Cuts and Jobs Act of 2017 [Member]          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Federal statutory income tax rate     21.00%    
Nevada Power Company [Member]          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Federal statutory income tax rate 21.00% 35.00% 21.00% 35.00%  
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Percent 3.00% 0.00% 3.00% 0.00%  
Effective Income Tax Rate Reconciliation Regulatory Differences 1.00% 0.00% 0.00% 0.00%  
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent (1.00%) 2.00% 0.00% 1.00%  
Effective income tax rate 24.00% 37.00% 24.00% 36.00%  
Nevada Power Company [Member] | Tax Cuts and Jobs Act of 2017 [Member]          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Federal statutory income tax rate     21.00%