SEGMENT INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Operating Revenues	$ 1,013,151	$ 1,026,488	$ 2,329,011	$ 2,378,606
Energy Costs:
Fuel for power generation	217,954	171,316	554,181	400,936
Purchased power	205,970	205,686	498,141	486,894
Gas purchased for resale	7,383	5,382	62,277	46,491
Deferred energy	(55,270)	(29,036)	(221,022)	(30,285)
Energy efficiency program costs	16,042	32,584	38,486	76,609
Regulatory Disallowances	17,335	0	17,335	0
Total Costs	409,414	385,932	949,398	980,645
Gross Margin	603,737	640,556	1,379,613	1,397,961
Merger related costs	7,857	0	21,409	0
Other Cost and Expense, Operating	106,068	100,108	317,538	307,080
Utilities Operating Expense, Maintenance	17,176	19,014	66,128	76,190
Depreciation, Depletion and Amortization, Nonproduction	96,801	94,512	291,687	281,690
Taxes, Other	14,214	15,682	46,536	44,457
OPERATING INCOME	361,621	411,240	636,315	688,544
Unallocated Amount to Segment
Segment Reporting Information [Line Items]
Operating Revenues	3	4	10	12
Energy Costs:
Total Costs	0	0	0	0
Gross Margin	3	4	10	12
Merger related costs	229		1,394
Other Cost and Expense, Operating	830	608	2,747	2,382
Taxes, Other	57	45	318	282
OPERATING INCOME	(1,113)	(649)	(4,449)	(2,652)
NPC Electric
Segment Reporting Information [Line Items]
Operating Revenues	786,142	802,334	1,695,129	1,751,165
Energy Costs:
Fuel for power generation	163,127	123,992	412,904	285,799
Purchased power	172,582	171,687	383,386	388,494
Deferred energy	(45,381)	(22,685)	(154,484)	(15,461)
Energy efficiency program costs	13,998	28,492	32,807	65,466
Regulatory Disallowances	11,866		11,866
Total Costs	316,192	301,486	686,479	724,298
Gross Margin	469,950	500,848	1,008,650	1,026,867
Merger related costs	5,620		14,487
Other Cost and Expense, Operating	70,844	65,372	208,336	200,484
Utilities Operating Expense, Maintenance	11,208	12,533	45,172	52,594
Depreciation, Depletion and Amortization, Nonproduction	68,849	66,975	207,915	201,096
Taxes, Other	8,213	9,743	27,804	26,793
OPERATING INCOME	305,216	346,225	504,936	545,900
SPPC Total
Segment Reporting Information [Line Items]
Operating Revenues	227,006	224,150	633,872	627,429
Energy Costs:
Fuel for power generation	54,827	47,324	141,277	115,137
Purchased power	33,388	33,999	114,755	98,400
Gas purchased for resale	7,383	5,382	62,277	46,491
Deferred energy	(9,889)	(6,351)	(66,538)	(14,824)
Energy efficiency program costs	2,044	4,092	5,679	11,143
Regulatory Disallowances	5,469		5,469
Total Costs	93,222	84,446	262,919	256,347
Gross Margin	133,784	139,704	370,953	371,082
Merger related costs	2,008		5,528
Other Cost and Expense, Operating	34,394	34,128	106,455	104,214
Utilities Operating Expense, Maintenance	5,968	6,481	20,956	23,596
Depreciation, Depletion and Amortization, Nonproduction	27,952	27,537	83,772	80,594
Taxes, Other	5,944	5,894	18,414	17,382
OPERATING INCOME	57,518	65,664	135,828	145,296
SPPC Electric
Segment Reporting Information [Line Items]
Electric Domestic Regulated Revenue	213,463	212,073	560,392	549,886
Energy Costs:
Fuel for power generation	54,827	47,324	141,277	115,137
Purchased power	33,388	33,999	114,755	98,400
Deferred energy	(7,925)	(5,498)	(44,223)	(13,854)
Energy efficiency program costs	2,044	4,092	5,679	11,143
Regulatory Disallowances	5,469		5,469
Total Costs	87,803	79,917	222,957	210,826
Gross Margin	125,660	132,156	337,435	339,060
SPPC Gas
Segment Reporting Information [Line Items]
Gas Domestic Regulated Revenue	13,543	12,077	73,480	77,543
Energy Costs:
Gas purchased for resale	7,383	5,382	62,277	46,491
Deferred energy	(1,964)	(853)	(22,315)	(970)
Total Costs	5,419	4,529	39,962	45,521
Gross Margin	$ 8,124	$ 7,548	$ 33,518	$ 32,022