Summary of Significant Accounting Policies - NPC - Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts [Line Items]
Beginning balance	$ 40
Ending balance	42	$ 40
Nevada Power Company [Member] | SEC Schedule, 12-09, Allowance, Credit Loss [Member]
Allowance for Doubtful Accounts [Line Items]
Beginning balance	16	12	$ 13
Charged to operating costs and expenses, net	15	15	16
Write-offs, net	15	11	17
Ending balance	$ 16	$ 16	$ 12