RETIREMENT PLAN AND POST-RETIREMENT BENEFITS, Components of Net Periodic Cost and Expected Benefit Payments (Details) (USD $)	3 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 Nevada Power Company [Member]	Sep. 30, 2011 Nevada Power Company [Member]	Sep. 30, 2012 Nevada Power Company [Member] United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 Nevada Power Company [Member] United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 Nevada Power Company [Member] United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 Nevada Power Company [Member] United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 Nevada Power Company [Member] United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 Nevada Power Company [Member] United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 Nevada Power Company [Member] United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 Nevada Power Company [Member] United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 Sierra Pacific Power Company [Member]	Sep. 30, 2011 Sierra Pacific Power Company [Member]	Sep. 30, 2012 Sierra Pacific Power Company [Member] United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 Sierra Pacific Power Company [Member] United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 Sierra Pacific Power Company [Member] United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 Sierra Pacific Power Company [Member] United States Pension Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 Sierra Pacific Power Company [Member] United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 Sierra Pacific Power Company [Member] United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2012 Sierra Pacific Power Company [Member] United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]	Sep. 30, 2011 Sierra Pacific Power Company [Member] United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]
Net periodic benefit cost [Abstract]
Service cost				$ 4,406,000	$ 4,607,000	$ 13,220,000	$ 13,820,000	$ 595,000	$ 653,000	$ 1,787,000	$ 1,958,000			$ 2,358,000	$ 2,445,000	$ 7,072,000	$ 7,336,000	$ 350,000	$ 363,000	$ 1,050,000	$ 1,090,000			$ 1,695,000	$ 1,840,000	$ 5,086,000	$ 5,520,000	$ 227,000	$ 271,000	$ 682,000	$ 815,000
Interest cost				10,228,000	10,169,000	30,684,000	30,507,000	1,905,000	2,090,000	5,715,000	6,270,000			4,881,000	4,880,000	14,643,000	14,640,000	602,000	615,000	1,807,000	1,844,000			5,043,000	5,013,000	15,130,000	15,038,000	1,283,000	1,457,000	3,848,000	4,372,000
Expected return on plan assets				(12,447,000)	(12,192,000)	(37,341,000)	(36,575,000)	(1,563,000)	(1,596,000)	(4,690,000)	(4,789,000)			(6,237,000)	(6,169,000)	(18,711,000)	(18,508,000)	(592,000)	(590,000)	(1,775,000)	(1,769,000)			(5,937,000)	(5,741,000)	(17,813,000)	(17,223,000)	(941,000)	(976,000)	(2,823,000)	(2,929,000)
Amortization of:
Prior service (credit)/cost				(724,000)	(738,000)	(2,173,000)	(2,214,000)	(987,000)	(987,000)	(2,961,000)	(2,961,000)			(456,000)	(470,000)	(1,367,000)	(1,409,000)	229,000	229,000	687,000	687,000			(277,000)	(277,000)	(831,000)	(831,000)	(1,220,000)	(1,219,000)	(3,658,000)	(3,658,000)
Actuarial (gain)/loss				3,473,000	4,155,000	10,418,000	12,465,000	731,000	1,083,000	2,193,000	3,250,000			1,363,000	1,690,000	4,089,000	5,069,000	221,000	302,000	662,000	906,000			2,026,000	2,412,000	6,078,000	7,235,000	504,000	773,000	1,511,000	2,319,000
Total net benefit cost				4,936,000	6,001,000	14,808,000	18,003,000	681,000	1,243,000	2,044,000	3,728,000			1,909,000	2,376,000	5,726,000	7,128,000	810,000	919,000	2,431,000	2,758,000			2,550,000	3,247,000	7,650,000	9,739,000	(147,000)	306,000	(440,000)	919,000
Average percentage of net periodic costs capitalized [Abstract]
Average percentage of net periodic costs capitalized (in hundredths)		34.97%	33.26%									36.92%	37.18%									35.09%	31.22%
Company contributions to Pension and Other Postretirement benefits						15,000,000	10,000,000			7,100,000	0
DefinedBenefitPlanSettlementsBenefitObligation	$ 11,900,000	$ 7,300,000