SEGMENT INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Operating Revenues	$ 600,571	$ 1,026,488	$ 740,698	$ 611,420	$ 609,597	$ 1,017,796	$ 674,931	$ 640,983	$ 2,979,177	$ 2,943,307	$ 3,280,222
Energy Costs:
Fuel for power generation									565,381	680,585	821,484
Purchased power									603,999	633,874	648,881
Gas purchased for resale									74,352	125,155	137,702
Deferred energy									(106,728)	(83,333)	113,107
Energy efficiency program costs									96,677	43,537	0
Total Costs									1,233,681	1,399,818	1,721,174
Gross Margin									1,745,496	1,543,489	1,559,048
Other Cost and Expense, Operating									412,372	411,115	414,241
Utilities Operating Expense, Maintenance									109,725	103,307	104,567
Depreciation, Depletion and Amortization, Nonproduction									377,640	357,937	333,059
Taxes, Other									60,696	60,465	62,746
Operating Income	96,519	411,240	180,836	96,468	76,684	353,196	106,919	73,866	785,063	610,665	644,435
Assets	11,984,136				11,667,129				11,984,136	11,667,129	11,700,283
Capital expenditures									498,893	620,516	629,496
Unallocated Amount to Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues									16	15	23
Energy Costs:
Fuel for power generation									0	0	0
Purchased power									0	0	0
Gas purchased for resale									0	0	0
Deferred energy									0	0	0
Energy efficiency program costs									0
Total Costs									0	0	0
Gross Margin									16	15	23
Other Cost and Expense, Operating									4,960	4,289	3,760
Utilities Operating Expense, Maintenance									0	0	0
Depreciation, Depletion and Amortization, Nonproduction									0	0	0
Taxes, Other									438	290	235
Operating Income									(5,382)	(4,564)	(3,972)
Assets	26,718				8,523				26,718	8,523	20,822
Capital expenditures									0	0	(13,094)	[1]
NPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues									2,145,241	2,054,393	2,252,377
Energy Costs:
Fuel for power generation									407,687	498,487	588,419
Purchased power									472,715	477,226	505,239
Deferred energy									(67,976)	(16,300)	94,843
Energy efficiency program costs									81,845	37,292
Total Costs									894,271	996,705	1,188,501
Gross Margin									1,250,970	1,057,688	1,063,876
Other Cost and Expense, Operating									267,720	260,127	260,535
Utilities Operating Expense, Maintenance									74,364	64,320	71,759
Depreciation, Depletion and Amortization, Nonproduction									269,721	252,191	226,252
Taxes, Other									36,870	37,254	37,918
Operating Income									602,295	443,796	467,412
Assets	8,641,145				8,442,597				8,641,145	8,442,597	8,301,824
Capital expenditures									287,598	475,118	499,374
SPPC Total [Member]
Segment Reporting Information [Line Items]
Operating Revenues									833,920	888,899	1,027,822
Energy Costs:
Fuel for power generation									157,694	182,098	233,065
Purchased power									131,284	156,648	143,642
Gas purchased for resale									74,352	125,155	137,702
Deferred energy									(38,752)	(67,033)	18,264
Energy efficiency program costs									14,832	6,245
Total Costs									339,410	403,113	532,673
Gross Margin									494,510	485,786	495,149
Other Cost and Expense, Operating									139,692	146,699	149,946
Utilities Operating Expense, Maintenance									35,361	38,987	32,808
Depreciation, Depletion and Amortization, Nonproduction									107,919	105,746	106,807
Taxes, Other									23,388	22,921	24,593
Operating Income									188,150	171,433	180,995
Assets	3,316,273				3,216,009				3,316,273	3,216,009	3,377,637
Capital expenditures									211,295	145,398	143,216
SPPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues									725,874	716,417	836,879
Energy Costs:
Fuel for power generation									157,694	182,098	233,065
Purchased power									131,284	156,648	143,642
Deferred energy									(26,369)	(65,445)	8,475
Energy efficiency program costs									14,832	6,245
Total Costs									277,441	279,546	385,182
Gross Margin									448,433	436,871	451,697
Assets	2,919,362				2,844,718				2,919,362	2,844,718	3,047,184
Capital expenditures									190,702	132,083	131,579
SPPC Gas [Member]
Segment Reporting Information [Line Items]
Operating Revenues									108,046	172,482	190,943
Energy Costs:
Gas purchased for resale									74,352	125,155	137,702
Deferred energy									(12,383)	(1,588)	9,789
Total Costs									61,969	123,567	147,491
Gross Margin									46,077	48,915	43,452
Assets	318,901				308,272				318,901	308,272	296,810
Capital expenditures									20,593	13,315	11,637
Intersegment Elimination [Member]
Energy Costs:
Assets	$ 78,010				$ 63,019				$ 78,010	$ 63,019	$ 33,643

[1] The capital expenditures for NVE Other at December 31, 2010 includes $13.1 million proceeds from the sale of assets between SPPC and SPCOM.