INCOME TAXES (BENEFITS) (Details6) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Unrecognized tax benefits [Roll Forward]
Begining Balance		$ 35.7	$ 38.2	$ 93.9
Increase in current period tax positions		0.5	0.3	3.3
Increase in prior period tax positions		0.2	0.1	11.8
Decrease in prior period tax positions	(64.4)	2.3	2.9	70.8
Ending Balance		34.1	35.7	38.2
Sierra Pacific Power Company [Member]
Unrecognized tax benefits [Roll Forward]
Begining Balance		10.2	10.5	40.1
Increase in current period tax positions		0.1	0.2	0.5
Increase in prior period tax positions		0.1	0.1	2.5
Decrease in prior period tax positions	(32.2)	0.6	0.6	32.6
Ending Balance		9.8	10.2	10.5
Nevada Power Company [Member]
Unrecognized tax benefits [Roll Forward]
Begining Balance		25.5	26.6	48.5
Increase in current period tax positions		0.4	0.1	2.8
Increase in prior period tax positions		0.1	0.1	9.2
Decrease in prior period tax positions	(32.0)	1.7	1.3	33.9
Ending Balance		$ 24.3	$ 25.5	$ 26.6