JOINTLY OWNED FACILITIES (Details) (Jointly Owned Electricity Generation Plant [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 MW	Dec. 31, 2010
NPC [Member]
Jointly Owned Utility Plant Interests [Line Items]
Plant in Service	$ 689,275	$ 666,231
Accumulated Depreciation	296,446	286,567
Net Plant in Service	392,829	379,664
CWIP	8,920	21,200
NPC [Member] | Navajo Generating Station [Member]
Jointly Owned Utility Plant Interests [Line Items]
% Owned (in hundredths)	11.30%	11.30%
Plant in Service	270,448	249,646
Accumulated Depreciation	148,582	141,326
Net Plant in Service	121,866	108,320
CWIP	1,117	1
NPC [Member] | Reid Gardner Generating Station No. 4 [Member]
Jointly Owned Utility Plant Interests [Line Items]
% Owned (in hundredths)	32.20%	32.20%
Plant in Service	171,485	165,795
Accumulated Depreciation	97,042	98,047
Net Plant in Service	74,443	67,748
CWIP	7,600	21,016
Proportionate share of base load capacity (in megawatts)	25
Proportionate share of peaking capacity (in megawatts)	232
NPC [Member] | Silverhawk Generating Station [Member]
Jointly Owned Utility Plant Interests [Line Items]
% Owned (in hundredths)	75.00%	75.00%
Plant in Service	247,342	250,790
Accumulated Depreciation	50,822	47,194
Net Plant in Service	196,520	203,596
CWIP	203	183
SPPC [Member] | Valmy Generating Station [Member]
Jointly Owned Utility Plant Interests [Line Items]
% Owned (in hundredths)	50.00%	50.00%
Plant in Service	331,753	313,378
Accumulated Depreciation	215,642	210,165
Net Plant in Service	116,111	103,213
CWIP	$ 6,682	$ 5,605
CDWR [Member] | Reid Gardner Generating Station No. 4 [Member]
Jointly Owned Utility Plant Interests [Line Items]
% Owned (in hundredths)	67.80%
Idaho Power Company [Member] | Valmy Generating Station [Member]
Jointly Owned Utility Plant Interests [Line Items]
% Owned (in hundredths)	50.00%