SEGMENT INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Operating Revenues	$ 611,420	$ 640,983
Energy Costs:
Fuel for power generation	117,035	146,338
Purchased power	117,116	135,016
Gas purchased for resale	31,617	52,632
Deferred energy	(11,739)	(1,952)
Energy efficiency program costs	19,425	0
Total Costs	273,454	332,034
Gross Margin	337,966	308,949
Other Cost and Expense, Operating	103,601	105,974
Utilities Operating Expense, Maintenance	32,526	29,762
Depreciation, Depletion and Amortization, Nonproduction	90,862	83,102
Taxes, Other	14,509	16,245
Operating Income	96,468	73,866
Unallocated Amount to Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	4	4
Energy Costs:
Total Costs	0
Gross Margin	4	4
Other Cost and Expense, Operating	707	657
Utilities Operating Expense, Maintenance	0	0
Taxes, Other	192	163
Operating Income	(895)	(816)
NPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	395,688	390,068
Energy Costs:
Fuel for power generation	80,549	101,070
Purchased power	81,531	95,566
Deferred energy	2,171	6,730
Energy efficiency program costs	15,774	0
Total Costs	180,025	203,366
Gross Margin	215,663	186,702
Other Cost and Expense, Operating	66,462	65,101
Utilities Operating Expense, Maintenance	23,073	22,337
Depreciation, Depletion and Amortization, Nonproduction	64,990	57,673
Taxes, Other	8,454	10,058
Operating Income	52,684	31,533
SPPC Total [Member]
Segment Reporting Information [Line Items]
Operating Revenues	215,728	250,911
Energy Costs:
Fuel for power generation	36,486	45,268
Purchased power	35,585	39,450
Gas purchased for resale	31,617	52,632
Deferred energy	(13,910)	(8,682)
Energy efficiency program costs	3,651	0
Total Costs	93,429	128,668
Gross Margin	122,299	122,243
Other Cost and Expense, Operating	36,432	40,216
Utilities Operating Expense, Maintenance	9,453	7,425
Depreciation, Depletion and Amortization, Nonproduction	25,872	25,429
Taxes, Other	5,863	6,024
Operating Income	44,679	43,149
SPPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	169,806	178,617
Energy Costs:
Fuel for power generation	36,486	45,268
Purchased power	35,585	39,450
Deferred energy	(12,670)	(11,931)
Energy efficiency program costs	3,651	0
Total Costs	63,052	72,787
Gross Margin	106,754	105,830
SPPC Gas [Member]
Segment Reporting Information [Line Items]
Operating Revenues	45,922	72,294
Energy Costs:
Gas purchased for resale	31,617	52,632
Deferred energy	(1,240)	3,249
Energy efficiency program costs		0
Total Costs	30,377	55,881
Gross Margin	$ 15,545	$ 16,413