INCOME TAXES (BENEFITS) (Details5) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of unrecognized tax benefits [Abstract]
Unrecognized tax benefits	$ 6.6	$ 34.1	$ 35.7	$ 38.2
Of the total, amounts related to tax positions that, if recognized, in future years would: Increase the effective tax rate	2.4	5.6	4.8
Sierra Pacific Power Company [Member]
Summary of unrecognized tax benefits [Abstract]
Unrecognized tax benefits	2.8	9.8	10.2	10.5
Of the total, amounts related to tax positions that, if recognized, in future years would: Increase the effective tax rate	0.8	1.8	1.6
Nevada Power Company [Member]
Summary of unrecognized tax benefits [Abstract]
Unrecognized tax benefits	3.8	24.3	25.5	26.6
Of the total, amounts related to tax positions that, if recognized, in future years would: Increase the effective tax rate	$ 1.6	$ 3.8	$ 3.2