INCOME TAXES (BENEFITS) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Current and other [Abstract]
Federal	$ (18.9)	$ (1.3)	$ (15.4)
State	(0.4)	0.1	1.0
Total current and other	(19.3)	(1.2)	(14.4)
Deferred [Abstract]
Federal	189.3	91.7	132.7
State	0	(0.1)	(0.1)
Total deferred	189.3	91.6	132.6
Amortization of excess deferred taxes	(0.8)	(0.4)	(1.1)
Investment tax credits	(2.6)	(3.1)	(3.3)
Total provision for income taxes	166.6	86.9	113.8
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]
Current and other [Abstract]
Federal	(6.8)	(0.1)	1.1
State	(0.4)	0.1	0.9
Total current and other	(7.2)	0	2.0
Deferred [Abstract]
Federal	49.1	33.2	42.0
State	0	0.2	(0.9)
Total deferred	49.1	33.4	41.1
Amortization of excess deferred taxes	(0.5)	(0.3)	(0.8)
Investment tax credits	(1.2)	(1.9)	(1.9)
Total provision for income taxes	40.2	31.2	40.4
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]
Current and other [Abstract]
Federal	(12.2)	(1.1)	(0.9)
State	0	0	0
Total current and other	(12.2)	(1.1)	(0.9)
Deferred [Abstract]
Federal	152.0	73.4	93.6
State	0	(0.3)	0.7
Total deferred	152.0	73.1	94.3
Amortization of excess deferred taxes	(0.3)	(0.1)	(0.2)
Investment tax credits	(1.4)	(1.2)	(1.4)
Total provision for income taxes	138.1	70.7	91.8
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