Summary of Significant Accounting Policies - SPPC - Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts [Line Items]
Beginning balance	$ 33
Ending balance	40	$ 33
Sierra Pacific Power Company [Member] | Allowance for Doubtful Accounts [Member]
Allowance for Doubtful Accounts [Line Items]
Beginning balance	2	1	$ 2
Charged to operating costs and expenses, net	2	2	1
Write-offs, net	2	1	2
Ending balance	$ 2	$ 2	$ 1