Risk Management and Hedging Activities - NPC - Derivative Contract Volumes (Details) - Commodity derivative [Member] MWh in Millions, Dth in Millions	Sep. 30, 2017 MWh Dth	Dec. 31, 2016 MWh Dth
Electricity purchases (in megawatt hours) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | MWh	9	5
Natural gas purchases (in decatherms) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | Dth	339	271
Nevada Power Company [Member] | Electricity purchases (in megawatt hours) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | MWh	0	2
Nevada Power Company [Member] | Natural gas purchases (in decatherms) [Member]
Notional Amounts of Outstanding Derivative Positions [Line Items]
Derivative, nonmonetary notional amount | Dth	149	114