Summary of Significant Accounting Policies - SPPC - Income Taxes (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets and Liabilities [Line Items]
Regulatory asset, deferred income taxes	$ 1,600	$ 1,500
Regulatory liability, deferred income taxes	25	29
Sierra Pacific Power Company [Member]
Deferred Tax Assets and Liabilities [Line Items]
Regulatory asset, deferred income taxes	85	90
Regulatory liability, deferred income taxes	$ 6	$ 7