Summary of Significant Accounting Policies Summary of Significant Accounting Policies - NPC - 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
Nevada Power Company [Member]
Deferred Tax Assets and Liabilities [Line Items]
Regulatory asset, deferred income taxes	149	156
Regulatory liability, deferred income taxes	$ 10	$ 3