Income Taxes - NPC - Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Income Tax Expense (Benefit) [Line Items]
Current federal tax expense (benefit)	$ (573)	$ (687)	$ (816)
Deferred federal income tax expense (benefit)	26	1,098	1,274
Other Tax Expense (Benefit)	(7)	(8)	(8)
Income tax (benefit) expense	(554)	403	450
Nevada Power Company [Member]
Components of Income Tax Expense (Benefit) [Line Items]
Current federal tax expense (benefit)	62	68	0
Deferred federal income tax expense (benefit)	95	79	163
Other Tax Expense (Benefit)	(1)	(1)	(1)
Income tax (benefit) expense	$ 156	$ 146	$ 162