SEGMENT INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Operating Revenues	$ 731,638	$ 740,698	$ 1,315,860	$ 1,352,118
Energy Costs:
Fuel for power generation	188,979	112,585	336,227	229,620
Purchased power	170,861	164,092	292,171	281,208
Gas purchased for resale	17,274	9,492	54,894	41,109
Deferred energy	(86,687)	10,490	(165,752)	(1,249)
Energy efficiency program costs	12,599	24,600	22,444	44,025
Total Costs	303,026	321,259	539,984	594,713
Gross Margin	428,612	419,439	775,876	757,405
Merger related costs	13,552	0	13,552	0
Other Cost and Expense, Operating	106,798	103,371	211,470	206,972
Utilities Operating Expense, Maintenance	24,046	24,650	48,952	57,176
Depreciation, Depletion and Amortization, Nonproduction	98,884	96,316	194,886	187,178
Taxes, Other	15,846	14,266	32,322	28,775
Operating Income	169,486	180,836	274,694	277,304
Unallocated Amount to Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues	4	4	7	8
Energy Costs:
Total Costs	0	0	0	0
Gross Margin	4	4	7	8
Merger related costs	1,165		1,165
Other Cost and Expense, Operating	442	1,067	1,917	1,774
Taxes, Other	39	45	261	237
Operating Income	(1,642)	(1,108)	(3,336)	(2,003)
NPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	537,124	553,143	908,987	948,831
Energy Costs:
Fuel for power generation	144,246	81,258	249,777	161,807
Purchased power	129,396	135,276	210,804	216,807
Deferred energy	(63,748)	5,053	(109,103)	7,224
Energy efficiency program costs	10,842	21,200	18,809	36,974
Total Costs	220,736	242,787	370,287	422,812
Gross Margin	316,388	310,356	538,700	526,019
Merger related costs	8,867		8,867
Other Cost and Expense, Operating	70,100	68,650	137,492	135,112
Utilities Operating Expense, Maintenance	15,889	16,988	33,964	40,061
Depreciation, Depletion and Amortization, Nonproduction	70,405	69,131	139,066	134,121
Taxes, Other	9,632	8,596	19,591	17,050
Operating Income	141,495	146,991	199,720	199,675
SPPC Total [Member]
Segment Reporting Information [Line Items]
Operating Revenues	194,510	187,551	406,866	403,279
Energy Costs:
Fuel for power generation	44,733	31,327	86,450	67,813
Purchased power	41,465	28,816	81,367	64,401
Gas purchased for resale	17,274	9,492	54,894	41,109
Deferred energy	(22,939)	5,437	(56,649)	(8,473)
Energy efficiency program costs	1,757	3,400	3,635	7,051
Total Costs	82,290	78,472	169,697	171,901
Gross Margin	112,220	109,079	237,169	231,378
Merger related costs	3,520		3,520
Other Cost and Expense, Operating	36,256	33,654	72,061	70,086
Utilities Operating Expense, Maintenance	8,157	7,662	14,988	17,115
Depreciation, Depletion and Amortization, Nonproduction	28,479	27,185	55,820	53,057
Taxes, Other	6,175	5,625	12,470	11,488
Operating Income	29,633	34,953	78,310	79,632
SPPC Electric [Member]
Segment Reporting Information [Line Items]
Operating Revenues	174,302	168,007	346,929	337,813
Energy Costs:
Fuel for power generation	44,733	31,327	86,450	67,813
Purchased power	41,465	28,816	81,367	64,401
Deferred energy	(16,963)	4,314	(36,298)	(8,356)
Energy efficiency program costs	1,757	3,400	3,635	7,051
Total Costs	70,992	67,857	135,154	130,909
Gross Margin	103,310	100,150	211,775	206,904
SPPC Gas [Member]
Segment Reporting Information [Line Items]
Operating Revenues	20,208	19,544	59,937	65,466
Energy Costs:
Gas purchased for resale	17,274	9,492	54,894	41,109
Deferred energy	(5,976)	1,123	(20,351)	(117)
Total Costs	11,298	10,615	34,543	40,992
Gross Margin	$ 8,910	$ 8,929	$ 25,394	$ 24,474