Summary of Significant Accounting Policies - NPC - Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Allowance for Doubtful Accounts [Line Items]
Beginning balance	$ 37
Ending balance	31	$ 37
Nevada Power Company [Member] | Allowance for Doubtful Accounts [Member]
Allowance for Doubtful Accounts [Line Items]
Beginning balance	14	8	$ 8
Charged to operating costs and expenses, net	16	14	15
Write-offs, net	17	8	15
Ending balance	$ 13	$ 14	$ 8