Fair Value Measurements - NPC - Level 3 (Details) - Nevada Power Company [Member] - Commodity [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ (8)	$ (14)	$ (3)	$ (14)
Fair Value, Measurements with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included In Regulatory Assets and Liabilities, Net	3	1	8	2
Fair Value, Measurements With Unobservable Inputs Reconciliation, Recurring Basis, Assets and Liability, Net, Settlements	(2)	(11)	(2)	(12)
Ending balance	$ (9)	$ (4)	$ (9)	$ (4)