INCOME TAXES (BENEFITS) (Details3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Components of Deferred Tax Assets [Abstract]
Net operating loss and credit carryovers	$ 470.8	$ 173.8
Employee benefit plans	58.4	66.3
Customer advances	17.6	25.2
Gross-ups received on CIAC & customer advances	20.3	26.2
Deferred revenues	18.5	8.0
Deferred energy	49.9	70.5
Reserves	13.4	11.3
Other	17.5	27.9
Total deferred tax assets - Non-Regulatory	666.4	409.2
Regulatory deferred tax assets
Excess deferred income taxes	8.7	9.2
Unamortized investment tax credit	8.7	10.3
Deferred Tax Assets, Regulatory Assets and Liabilities	17.4	19.5
Total deferred tax assets before valuation allowance	683.8	428.7
Valuation allowance	1.2	1.5
Total deferred tax assets after valuation allowance	682.6	427.2
Deferred tax liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,380.7	1,004.2
Deferred Conservation Programs	83.2	78.8
Regulatory assets	137.1	166.1
Other	32.0	36.6
Total deferred tax liabilities - Non-Regulatory	1,633.0	1,285.7
Regulatory deferred tax liabilities
Tax benefits flowed through to customers - property	115.2	116.9
Tax benefits flowed through to customers - goodwill	136.0	140.2
Deferred Tax Liabilities, Regulatory Assets and Liabilities	251.2	257.1
Total deferred tax liabilities, including regulatory deferred tax liabilities	1,884.2	1,542.8
Deferred tax assets and liabilities net non regulatory	967.8	878.0
Deferred tax assets liabilities net regulatory	233.8	237.6
Deferred Tax Assets (Liabilities), Net	1,201.6	1,115.6
Nevada Power Company [Member]
Components of Deferred Tax Assets [Abstract]
Net operating loss and credit carryovers	353.7	82.5
Employee benefit plans	21.2	25.7
Customer advances	10.5	12.3
Gross-ups received on CIAC & customer advances	15.3	19.4
Deferred revenues	15.1	3.5
Deferred energy	20.0	18.8
Reserves	9.6	9.9
Other	10.5	19.3
Total deferred tax assets - Non-Regulatory	455.9	191.4
Regulatory deferred tax assets
Excess deferred income taxes	2.5	2.7
Unamortized investment tax credit	3.3	3.9
Deferred Tax Assets, Regulatory Assets and Liabilities	5.8	6.6
Total deferred tax assets before valuation allowance	461.7	198.0
Valuation allowance	1.2	1.5
Total deferred tax assets after valuation allowance	460.5	196.5
Deferred tax liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,015.1	661.7
Deferred Conservation Programs	63.0	58.6
Regulatory assets	94.1	112.9
Other	19.4	21.1
Total deferred tax liabilities - Non-Regulatory	1,191.6	854.3
Regulatory deferred tax liabilities
Tax benefits flowed through to customers - property	93.0	86.3
Tax benefits flowed through to customers - goodwill	85.0	87.7
Deferred Tax Liabilities, Regulatory Assets and Liabilities	178.0	174.0
Total deferred tax liabilities, including regulatory deferred tax liabilities	1,369.6	1,028.3
Deferred tax assets and liabilities net non regulatory	736.9	664.3
Deferred tax assets liabilities net regulatory	172.2	167.5
Deferred Tax Assets (Liabilities), Net	909.1	831.8
Sierra Pacific Power Company [Member]
Components of Deferred Tax Assets [Abstract]
Net operating loss and credit carryovers	62.5	46.3
Employee benefit plans	26.5	34.8
Customer advances	7.1	12.9
Gross-ups received on CIAC & customer advances	5.0	6.8
Deferred revenues	3.4	4.5
Deferred energy	29.9	51.7
Reserves	2.5	1.4
Other	6.3	7.9
Total deferred tax assets - Non-Regulatory	143.2	166.3
Regulatory deferred tax assets
Excess deferred income taxes	6.2	6.5
Unamortized investment tax credit	5.4	6.5
Deferred Tax Assets, Regulatory Assets and Liabilities	11.6	13.0
Total deferred tax assets before valuation allowance	154.8	179.3
Valuation allowance	0	0
Total deferred tax assets after valuation allowance	154.8	179.3
Deferred tax liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	371.6	348.9
Deferred Conservation Programs	20.2	20.1
Regulatory assets	44.2	54.3
Other	12.1	15.1
Total deferred tax liabilities - Non-Regulatory	448.1	438.4
Regulatory deferred tax liabilities
Tax benefits flowed through to customers - property	22.3	30.6
Tax benefits flowed through to customers - goodwill	50.9	52.5
Deferred Tax Liabilities, Regulatory Assets and Liabilities	73.2	83.1
Total deferred tax liabilities, including regulatory deferred tax liabilities	521.3	521.5
Deferred tax assets and liabilities net non regulatory	304.9	272.1
Deferred tax assets liabilities net regulatory	61.6	70.1
Deferred Tax Assets (Liabilities), Net	$ 366.5	$ 342.2