SEGMENT INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Operating Revenues	$ 740,698	$ 674,931	$ 1,352,118	$ 1,315,914
Energy Costs:
Fuel for power generation	112,585	156,803	229,620	303,141
Purchased power	164,092	160,308	281,208	295,324
Gas purchased for resale	9,492	24,984	41,109	77,616
Deferred energy	10,490	(8,106)	(1,249)	(10,058)
Energy efficiency program costs	24,600	0	44,025	0
Total Costs	321,259	333,989	594,713	666,023
Gross Margin	419,439	340,942	757,405	649,891
Other Cost and Expense, Operating	103,371	97,547	206,972	203,521
Utilities Operating Expense, Maintenance	24,650	32,186	57,176	61,948
Depreciation, Depletion and Amortization, Nonproduction	96,316	89,606	187,178	172,708
Taxes, Other	14,266	14,684	28,775	30,929
Operating Income	180,836	106,919	277,304	180,785
Unallocated Amount to Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	4	3	8	7
Energy Costs:
Total Costs	0	0	0	0
Gross Margin	4	3	8	7
Other Cost and Expense, Operating	1,067	925	1,774	1,582
Taxes, Other	45	39	237	202
Operating Income	(1,108)	(961)	(2,003)	(1,777)
NPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	553,143	473,898	948,831	863,966
Energy Costs:
Fuel for power generation	81,258	114,744	161,807	215,814
Purchased power	135,276	122,408	216,807	217,974
Deferred energy	5,053	2,350	7,224	9,080
Energy efficiency program costs	21,200	0	36,974	0
Total Costs	242,787	239,502	422,812	442,868
Gross Margin	310,356	234,396	526,019	421,098
Other Cost and Expense, Operating	68,650	61,935	135,112	127,036
Utilities Operating Expense, Maintenance	16,988	19,325	40,061	41,662
Depreciation, Depletion and Amortization, Nonproduction	69,131	61,913	134,121	119,586
Taxes, Other	8,596	9,046	17,050	19,104
Operating Income	146,991	82,177	199,675	113,710
SPPC Total [Member]
Segment Reporting Information [Line Items]
Operating Revenues	187,551	201,030	403,279	451,941
Energy Costs:
Fuel for power generation	31,327	42,059	67,813	87,327
Purchased power	28,816	37,900	64,401	77,350
Gas purchased for resale	9,492	24,984	41,109	77,616
Deferred energy	5,437	(10,456)	(8,473)	(19,138)
Energy efficiency program costs	3,400	0	7,051	0
Total Costs	78,472	94,487	171,901	223,155
Gross Margin	109,079	106,543	231,378	228,786
Other Cost and Expense, Operating	33,654	34,687	70,086	74,903
Utilities Operating Expense, Maintenance	7,662	12,861	17,115	20,286
Depreciation, Depletion and Amortization, Nonproduction	27,185	27,693	53,057	53,122
Taxes, Other	5,625	5,599	11,488	11,623
Operating Income	34,953	25,703	79,632	68,852
SPPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	168,007	164,582	337,813	343,199
Energy Costs:
Fuel for power generation	31,327	42,059	67,813	87,327
Purchased power	28,816	37,900	64,401	77,350
Deferred energy	4,314	(11,898)	(8,356)	(23,829)
Energy efficiency program costs	3,400	0	7,051	0
Total Costs	67,857	68,061	130,909	140,848
Gross Margin	100,150	96,521	206,904	202,351
SPPC Gas [Member]
Segment Reporting Information [Line Items]
Operating Revenues	19,544	36,448	65,466	108,742
Energy Costs:
Gas purchased for resale	9,492	24,984	41,109	77,616
Deferred energy	1,123	1,442	(117)	4,691
Total Costs	10,615	26,426	40,992	82,307
Gross Margin	$ 8,929	$ 10,022	$ 24,474	$ 26,435