INCOME TAXES (BENEFITS) (Details6) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unrecognized tax benefits [Roll Forward]
Begining Balance	$ 34.1	$ 35.7	$ 38.2
Increase in current period tax positions	1.1	0.5	0.3
Increase in prior period tax positions	1.6	0.2	0.1
Decrease in prior period tax positions	(30.2)	(2.3)	(2.9)
Ending Balance	6.6	34.1	35.7
Years no longer subject to examination	years before 2008
Sierra Pacific Power Company [Member]
Unrecognized tax benefits [Roll Forward]
Begining Balance	9.8	10.2	10.5
Increase in current period tax positions	0.3	0.1	0.2
Increase in prior period tax positions	1.7	0.1	0.1
Decrease in prior period tax positions	(9.0)	(0.6)	(0.6)
Ending Balance	2.8	9.8	10.2
Nevada Power Company [Member]
Unrecognized tax benefits [Roll Forward]
Begining Balance	24.3	25.5	26.6
Increase in current period tax positions	0.8	0.4	0.1
Increase in prior period tax positions	(0.1)	0.1	0.1
Decrease in prior period tax positions	(21.2)	(1.7)	(1.3)
Ending Balance	$ 3.8	$ 24.3	$ 25.5