INCOME TAXES (BENEFITS) (Details3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Components of Deferred Tax Assets [Abstract]
Net operating loss and credit carryovers	$ 384.9	$ 470.8
Employee benefit plans	92.0	58.4
Customer advances	17.0	17.6
Gross-ups received on CIAC & customer advances	19.4	20.3
Deferred revenues	15.2	18.5
Deferred energy	17.5	49.9
Reserves	15.0	13.4
Other	7.6	17.5
Total deferred tax assets - Non-Regulatory	568.6	666.4
Regulatory deferred tax assets
Excess deferred income taxes	7.8	8.7
Unamortized investment tax credit	7.3	8.7
Deferred Tax Assets, Regulatory Assets and Liabilities	15.1	17.4
Total deferred tax assets before valuation allowance	583.7	683.8
Valuation allowance	(1.5)	(1.2)
Total deferred tax assets after valuation allowance	582.2	682.6
Deferred tax liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,481.7	1,380.7
Deferred Conservation Programs	42.4	83.2
Regulatory assets	168.4	137.1
Other	30.4	32.0
Total deferred tax liabilities - Non-Regulatory	1,722.9	1,633.0
Regulatory deferred tax liabilities
Tax benefits flowed through to customers - property	137.1	147.2
Tax benefits flowed through to customers - goodwill	132.6	136.0
Deferred Tax Liabilities, Regulatory Assets and Liabilities	269.7	283.2
Total deferred tax liabilities, including regulatory deferred tax liabilities	1,992.6	1,916.2
Deferred tax assets and liabilities net non regulatory	1,155.8	967.8
Deferred tax assets liabilities net regulatory	254.6	265.8
Deferred Tax Assets (Liabilities), Net	1,410.4	1,233.6
Nevada Power Company [Member]
Components of Deferred Tax Assets [Abstract]
Net operating loss and credit carryovers	264.2	353.7
Employee benefit plans	35.5	21.2
Customer advances	10.8	10.5
Gross-ups received on CIAC & customer advances	13.8	15.3
Deferred revenues	12.7	15.1
Deferred energy	(0.3)	20.0
Reserves	10.5	9.6
Other	3.2	10.5
Total deferred tax assets - Non-Regulatory	350.4	455.9
Regulatory deferred tax assets
Excess deferred income taxes	2.2	2.5
Unamortized investment tax credit	2.5	3.3
Deferred Tax Assets, Regulatory Assets and Liabilities	4.7	5.8
Total deferred tax assets before valuation allowance	355.1	461.7
Valuation allowance	(1.5)	(1.2)
Total deferred tax assets after valuation allowance	353.6	460.5
Deferred tax liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,071.1	1,015.1
Deferred Conservation Programs	32.4	63.0
Regulatory assets	115.0	94.1
Other	19.1	19.4
Total deferred tax liabilities - Non-Regulatory	1,237.6	1,191.6
Regulatory deferred tax liabilities
Tax benefits flowed through to customers - property	85.9	93.0
Tax benefits flowed through to customers - goodwill	83.3	85.0
Deferred Tax Liabilities, Regulatory Assets and Liabilities	169.2	178.0
Total deferred tax liabilities, including regulatory deferred tax liabilities	1,406.8	1,369.6
Deferred tax assets and liabilities net non regulatory	888.7	736.9
Deferred tax assets liabilities net regulatory	164.5	172.2
Deferred Tax Assets (Liabilities), Net	1,053.2	909.1
Sierra Pacific Power Company [Member]
Components of Deferred Tax Assets [Abstract]
Net operating loss and credit carryovers	61.1	62.5
Employee benefit plans	38.5	26.5
Customer advances	6.2	7.1
Gross-ups received on CIAC & customer advances	5.6	5.0
Deferred revenues	2.5	3.4
Deferred energy	17.8	29.9
Reserves	2.2	2.5
Other	3.5	6.3
Total deferred tax assets - Non-Regulatory	137.4	143.2
Regulatory deferred tax assets
Excess deferred income taxes	5.6	6.2
Unamortized investment tax credit	4.8	5.4
Deferred Tax Assets, Regulatory Assets and Liabilities	10.4	11.6
Total deferred tax assets before valuation allowance	147.8	154.8
Valuation allowance	0	0
Total deferred tax assets after valuation allowance	147.8	154.8
Deferred tax liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	416.2	371.6
Deferred Conservation Programs	10.0	20.2
Regulatory assets	54.2	44.2
Other	10.8	12.1
Total deferred tax liabilities - Non-Regulatory	491.2	448.1
Regulatory deferred tax liabilities
Tax benefits flowed through to customers - property	51.2	54.3
Tax benefits flowed through to customers - goodwill	49.3	50.9
Deferred Tax Liabilities, Regulatory Assets and Liabilities	100.5	105.2
Total deferred tax liabilities, including regulatory deferred tax liabilities	591.7	553.3
Deferred tax assets and liabilities net non regulatory	353.8	304.9
Deferred tax assets liabilities net regulatory	90.1	93.6
Deferred Tax Assets (Liabilities), Net	$ 443.9	$ 398.5