QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Data [Line Items]
Operating Revenues	$ 609,597	$ 1,017,796	$ 674,931	$ 640,983	$ 655,011	$ 1,128,039	$ 782,683	$ 714,489	$ 2,943,307	$ 3,280,222	$ 3,585,798
Operating Income	76,684	353,196	106,919	73,866	103,435	343,364	124,730	72,906	610,665	644,435	564,083
Net Income (Loss)	(25,248)	173,462	12,888	2,330	14,213	177,546	36,946	(1,721)	163,432	226,984	182,936
Earnings Per Share, Basic and Diluted [Abstract]
Net income per share - basic	$ (0.11)	$ 0.74	$ 0.05	$ 0.01	$ 0.06	$ 0.76	$ 0.16	$ (0.01)	$ 0.69	$ 0.97	$ 0.78
Net income per share - diluted	$ (0.11)	$ 0.73	$ 0.05	$ 0.01	$ 0.06	$ 0.75	$ 0.16	$ (0.01)	$ 0.69	$ 0.96	$ 0.78
Income Tax Expense (Benefit)									86,915	113,764	75,451
NPC [Member]
Quarterly Financial Data [Line Items]
Operating Revenues	391,513	798,914	473,898	390,068	416,233	870,950	539,395	425,799	2,054,393	2,252,377	2,423,377
Operating Income	33,759	296,327	82,177	31,533	55,450	288,163	93,670	30,129	443,796	467,412	396,362
Net Income (Loss)	(29,065)	154,608	16,063	(9,020)	9,557	158,928	29,784	(12,326)	132,586	185,943	134,284
Earnings Per Share, Basic and Diluted [Abstract]
Income Tax Expense (Benefit)									70,737	91,757	61,652
SPPC [Member]
Quarterly Financial Data [Line Items]
Operating Revenues	218,080	218,878	201,030	250,911	238,774	257,084	243,282	288,682	888,899	1,027,822	1,162,393
Operating Income	45,007	57,574	25,703	43,149	49,184	56,223	32,184	43,404	171,433	180,995	170,589
Net Income (Loss)	14,462	25,336	3,512	16,576	19,478	24,462	11,315	17,120	59,886	72,375	73,085
Earnings Per Share, Basic and Diluted [Abstract]
Income Tax Expense (Benefit)									$ 31,197	$ 40,404	$ 31,225