Summary of Significant Accounting Policies - NPC - Allowance for Doubtful Accounts (Details) - Nevada Power Company [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for doubtful accounts	$ 15	$ 16	$ 16
Accounts Receivable, Credit Loss Expense (Reversal)	13	12	15
Accounts Receivable, Allowance for Credit Loss, Writeoff	(9)	(13)	(15)
Allowance for doubtful accounts	$ 19	$ 15	$ 16