SEGMENT INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,026,488	$ 1,017,796	$ 2,378,606	$ 2,333,710
Energy Costs:
Fuel for power generation	171,316	216,779	400,936	519,920
Purchased power	205,686	223,348	486,894	518,672
Gas purchased for resale	5,382	10,137	46,491	87,753
Deferred energy	(29,036)	(33,620)	(30,285)	(43,678)
Energy efficiency program costs	32,584	23,047	76,609	23,047
Total Costs	385,932	439,691	980,645	1,105,714
Gross Margin	640,556	578,105	1,397,961	1,227,996
Other Cost and Expense, Operating	100,108	104,598	307,080	308,119
Utilities Operating Expense, Maintenance	19,014	11,369	76,190	73,317
Depreciation, Depletion and Amortization, Nonproduction	94,512	93,737	281,690	266,445
Taxes, Other	15,682	15,205	44,457	46,134
Operating Income	411,240	353,196	688,544	533,981
Unallocated Amount to Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	4	4	12	11
Energy Costs:
Total Costs	0	0	0	0
Gross Margin	4	4	12	11
Other Cost and Expense, Operating	608	661	2,382	2,243
Taxes, Other	45	48	282	250
Operating Income	(649)	(705)	(2,652)	(2,482)
NPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	802,334	798,914	1,751,165	1,662,880
Energy Costs:
Fuel for power generation	123,992	162,976	285,799	378,790
Purchased power	171,687	181,733	388,494	399,707
Deferred energy	(22,685)	(10,354)	(15,461)	(1,274)
Energy efficiency program costs	28,492	20,451	65,466	20,451
Total Costs	301,486	354,806	724,298	797,674
Gross Margin	500,848	444,108	1,026,867	865,206
Other Cost and Expense, Operating	65,372	68,004	200,484	195,040
Utilities Operating Expense, Maintenance	12,533	3,460	52,594	45,122
Depreciation, Depletion and Amortization, Nonproduction	66,975	67,212	201,096	186,798
Taxes, Other	9,743	9,105	26,793	28,209
Operating Income	346,225	296,327	545,900	410,037
SPPC Total [Member]
Segment Reporting Information [Line Items]
Operating Revenues	224,150	218,878	627,429	670,819
Energy Costs:
Fuel for power generation	47,324	53,803	115,137	141,130
Purchased power	33,999	41,615	98,400	118,965
Gas purchased for resale	5,382	10,137	46,491	87,753
Deferred energy	(6,351)	(23,266)	(14,824)	(42,404)
Energy efficiency program costs	4,092	2,596	11,143	2,596
Total Costs	84,446	84,885	256,347	308,040
Gross Margin	139,704	133,993	371,082	362,779
Other Cost and Expense, Operating	34,128	35,933	104,214	110,836
Utilities Operating Expense, Maintenance	6,481	7,909	23,596	28,195
Depreciation, Depletion and Amortization, Nonproduction	27,537	26,525	80,594	79,647
Taxes, Other	5,894	6,052	17,382	17,675
Operating Income	65,664	57,574	145,296	126,426
SPPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	212,073	202,263	549,886	545,462
Energy Costs:
Fuel for power generation	47,324	53,803	115,137	141,130
Purchased power	33,999	41,615	98,400	118,965
Deferred energy	(5,498)	(22,095)	(13,854)	(45,924)
Energy efficiency program costs	4,092	2,596	11,143	2,596
Total Costs	79,917	75,919	210,826	216,767
Gross Margin	132,156	126,344	339,060	328,695
SPPC Gas [Member]
Segment Reporting Information [Line Items]
Operating Revenues	12,077	16,615	77,543	125,357
Energy Costs:
Gas purchased for resale	5,382	10,137	46,491	87,753
Deferred energy	(853)	(1,171)	(970)	3,520
Total Costs	4,529	8,966	45,521	91,273
Gross Margin	$ 7,548	$ 7,649	$ 32,022	$ 34,084