SEGMENT INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Operating Revenues	$ 584,222	$ 611,420
Energy Costs:
Fuel for power generation	147,248	117,035
Purchased power	121,310	117,116
Gas purchased for resale	37,620	31,617
Deferred energy	(79,065)	(11,739)
Energy efficiency program costs	9,845	19,425
Total Costs	236,958	273,454
Gross Margin	347,264	337,966
Other Cost and Expense, Operating	104,672	103,601
Utilities Operating Expense, Maintenance	24,906	32,526
Depreciation, Depletion and Amortization, Nonproduction	96,002	90,862
Taxes, Other	16,476	14,509
Operating Income	105,208	96,468
Unallocated Amount to Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	3	4
Energy Costs:
Total Costs	0	0
Gross Margin	3	4
Other Cost and Expense, Operating	1,475	707
Taxes, Other	222	192
Operating Income	(1,694)	(895)
NPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	371,863	395,688
Energy Costs:
Fuel for power generation	105,531	80,549
Purchased power	81,408	81,531
Deferred energy	(45,355)	2,171
Energy efficiency program costs	7,967	15,774
Total Costs	149,551	180,025
Gross Margin	222,312	215,663
Other Cost and Expense, Operating	67,392	66,462
Utilities Operating Expense, Maintenance	18,075	23,073
Depreciation, Depletion and Amortization, Nonproduction	68,661	64,990
Taxes, Other	9,959	8,454
Operating Income	58,225	52,684
SPPC Total [Member]
Segment Reporting Information [Line Items]
Operating Revenues	212,356	215,728
Energy Costs:
Fuel for power generation	41,717	36,486
Purchased power	39,902	35,585
Gas purchased for resale	37,620	31,617
Deferred energy	(33,710)	(13,910)
Energy efficiency program costs	1,878	3,651
Total Costs	87,407	93,429
Gross Margin	124,949	122,299
Other Cost and Expense, Operating	35,805	36,432
Utilities Operating Expense, Maintenance	6,831	9,453
Depreciation, Depletion and Amortization, Nonproduction	27,341	25,872
Taxes, Other	6,295	5,863
Operating Income	48,677	44,679
SPPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	172,627	169,806
Energy Costs:
Fuel for power generation	41,717	36,486
Purchased power	39,902	35,585
Deferred energy	(19,335)	(12,670)
Energy efficiency program costs	1,878	3,651
Total Costs	64,162	63,052
Gross Margin	108,465	106,754
SPPC Gas [Member]
Segment Reporting Information [Line Items]
Operating Revenues	39,729	45,922
Energy Costs:
Gas purchased for resale	37,620	31,617
Deferred energy	(14,375)	(1,240)
Total Costs	23,245	30,377
Gross Margin	$ 16,484	$ 15,545