RETIREMENT PLAN AND POST-RETIREMENT BENEFITS, Components of Net Periodic Cost and Expected Benefit Payments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
United States Pension Plans of US Entity, Defined Benefit [Member]
Net periodic benefit cost [Abstract]
Service cost	$ 5,132	$ 4,406	$ 10,264	$ 8,813
Interest cost	9,303	10,228	18,607	20,456
Expected return on plan assets	(12,708)	(12,447)	(25,416)	(24,894)
Amortization of:
Prior service (credit)/cost	(720)	(724)	(1,441)	(1,448)
Actuarial (gain)/loss	4,797	3,473	9,594	6,945
Total net benefit cost	5,804	4,936	11,608	9,872
United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]
Net periodic benefit cost [Abstract]
Service cost	660	595	1,320	1,191
Interest cost	1,677	1,905	3,353	3,810
Expected return on plan assets	(1,687)	(1,563)	(3,373)	(3,126)
Amortization of:
Prior service (credit)/cost	(952)	(987)	(1,905)	(1,974)
Actuarial (gain)/loss	890	731	1,781	1,462
Total net benefit cost	588	681	1,176	1,363
Nevada Power Company [Member] | United States Pension Plans of US Entity, Defined Benefit [Member]
Net periodic benefit cost [Abstract]
Service cost	2,761	2,358	5,522	4,715
Interest cost	4,453	4,881	8,905	9,762
Expected return on plan assets	(6,270)	(6,237)	(12,540)	(12,474)
Amortization of:
Prior service (credit)/cost	(453)	(456)	(905)	(912)
Actuarial (gain)/loss	2,117	1,363	4,234	2,726
Total net benefit cost	2,608	1,909	5,216	3,817
Nevada Power Company [Member] | United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]
Net periodic benefit cost [Abstract]
Service cost	389	350	778	700
Interest cost	556	602	1,111	1,205
Expected return on plan assets	(631)	(592)	(1,261)	(1,183)
Amortization of:
Prior service (credit)/cost	(23)	229	(46)	458
Actuarial (gain)/loss	289	221	578	441
Total net benefit cost	580	810	1,160	1,621
Sierra Pacific Power Company [Member] | United States Pension Plans of US Entity, Defined Benefit [Member]
Net periodic benefit cost [Abstract]
Service cost	1,926	1,695	3,852	3,391
Interest cost	4,558	5,043	9,117	10,086
Expected return on plan assets	(6,162)	(5,937)	(12,324)	(11,875)
Amortization of:
Prior service (credit)/cost	(277)	(277)	(554)	(554)
Actuarial (gain)/loss	2,501	2,026	5,001	4,052
Total net benefit cost	2,546	2,550	5,092	5,100
Sierra Pacific Power Company [Member] | United States Postretirement Benefit Plans of US Entity, Defined Benefit [Member]
Net periodic benefit cost [Abstract]
Service cost	251	227	501	455
Interest cost	1,104	1,283	2,207	2,566
Expected return on plan assets	(1,022)	(941)	(2,043)	(1,882)
Amortization of:
Prior service (credit)/cost	(933)	(1,220)	(1,866)	(2,439)
Actuarial (gain)/loss	592	504	1,185	1,007
Total net benefit cost	$ (8)	$ (147)	$ (16)	$ (293)