INCOME TAXES (BENEFITS) (Details5) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Summary of unrecognized tax benefits [Abstract]
Unrecognized tax benefits	$ 34.1	$ 35.7	$ 38.2	$ 93.9
Of the total, amounts related to tax positions that, if recognized, in future years would: Increase the effective tax rate	5.6	4.8	4.5
Sierra Pacific Power Company [Member]
Summary of unrecognized tax benefits [Abstract]
Unrecognized tax benefits	9.8	10.2	10.5	40.1
Of the total, amounts related to tax positions that, if recognized, in future years would: Increase the effective tax rate	1.8	1.6	1.4
Nevada Power Company [Member]
Summary of unrecognized tax benefits [Abstract]
Unrecognized tax benefits	24.3	25.5	26.6	48.5
Of the total, amounts related to tax positions that, if recognized, in future years would: Increase the effective tax rate	$ 3.8	$ 3.2	$ 3.1