Summary of Significant Accounting Policies - SPPC - Income Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred Tax Assets and Liabilities [Line Items]
Regulatory asset, deferred income taxes	$ 1,500	$ 1,400
Regulatory liability, deferred income taxes	29	24
Sierra Pacific Power Company [Member]
Deferred Tax Assets and Liabilities [Line Items]
Regulatory asset, deferred income taxes	90	94
Regulatory liability, deferred income taxes	$ 7	$ 8