CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (USD $) In Thousands, except Share data	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	NPC [Member]	NPC [Member] Common Stock [Member]	NPC [Member] Additional Paid-in Capital [Member]	NPC [Member] Retained Earnings [Member]	NPC [Member] Accumulated Other Comprehensive Income (Loss) [Member]	SPPC [Member]	SPPC [Member] Common Stock [Member]	SPPC [Member] Additional Paid-in Capital [Member]	SPPC [Member] Retained Earnings [Member]	SPPC [Member] Accumulated Other Comprehensive Income (Loss) [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2008	$ 3,131,185	$ 234,317	$ 2,694,792	$ 208,436	$ (6,360)	$ 2,627,573	$ 1	$ 2,254,182	$ 377,061	$ (3,671)	$ 877,960	$ 4	$ 1,020,960	$ (140,686)	$ (2,318)
Balance (in shares) at Dec. 31, 2008		234,316,829					1,000					1,000
Net Income	182,936			182,936		134,284			134,284		73,085			73,085
Dividend Reinvestment and Employee Benefits	6,047	517	5,530
Dividend Reinvestment and Employee Benefits (in shares)		517,340
Tax benefit from stock options exercised	7		7			7		7
Capital contribution from parent											90,300		90,300
Change in market value of risk management assets and liabilities	0
Change in compensation retirement benefits liability and amortization	(128)				(128)	175				175	(87)				(87)
Dividends Declared	(96,125)			(96,125)		(112,000)			(112,000)		(32,000)			(32,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2009	3,223,922	234,834	2,700,329	295,247	(6,488)	2,650,039	1	2,254,189	399,345	(3,496)	1,009,258	4	1,111,260	(99,601)	(2,405)
Balance (in shares) at Dec. 31, 2009		234,834,169					1,000					1,000
Net Income	(1,721)					(12,326)					17,120
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Mar. 31, 2010
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2009	3,223,922	234,834	2,700,329	295,247	(6,488)	2,650,039	1		399,345	(3,496)	1,009,258	4	1,111,260	(99,601)	(2,405)
Balance (in shares) at Dec. 31, 2009		234,834,169					1,000					1,000
Net Income	226,984			226,984		185,943			185,943		72,375			72,375
Dividend Reinvestment and Employee Benefits	6,109	489	5,620
Dividend Reinvestment and Employee Benefits (in shares)		488,384
Tax benefit from stock options exercised	32		32								2		2
Change in market value of risk management assets and liabilities	0
Change in compensation retirement benefits liability and amortization	(403)				(403)	(380)				(380)	(215)				(215)
Common stock issuance costs	(27)		(27)
Dividends Declared	(105,799)			(105,799)		(74,000)			(74,000)		(108,000)			(108,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2010	3,350,818	235,323	2,705,954	416,432	(6,891)	2,761,632	1		511,288	(3,876)	973,420	4	1,111,262	(135,226)	(2,620)
Balance (in shares) at Dec. 31, 2010	235,322,553	235,322,553				1,000	1,000				1,000	1,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Sep. 30, 2010
Net Income	14,213					9,557					19,478
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2010	3,350,818					2,761,632	1				973,420	4
Balance (in shares) at Dec. 31, 2010	235,322,553					1,000	1,000				1,000	1,000
Net Income	2,330					(9,020)					16,576
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Mar. 31, 2011
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2010	3,350,818	235,323	2,705,954	416,432	(6,891)	2,761,632	1	2,254,219	511,288	(3,876)	973,420	4		(135,226)	(2,620)
Balance (in shares) at Dec. 31, 2010	235,322,553	235,322,553				1,000	1,000				1,000	1,000
Net Income	163,432			163,432		132,586			132,586		59,886			59,886
Dividend Reinvestment and Employee Benefits	8,459	677	7,782
Dividend Reinvestment and Employee Benefits (in shares)		677,197
Tax benefit from stock options exercised						30		30
Capital contribution from parent						54,000		54,000
Change in market value of risk management assets and liabilities	(686)				(686)
Change in compensation retirement benefits liability and amortization	(357)				(357)	(241)				(241)	1,236				1,236
Dividends Declared				(115,587)		(99,000)			(99,000)		(60,000)			(60,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2011	3,406,079	236,000	2,713,736	464,277	(7,934)	2,848,977	1	2,308,219	544,874	(4,117)	974,542	4	1,111,262	(135,340)	(1,384)
Balance (in shares) at Dec. 31, 2011	235,999,750	235,999,750				1,000	1,000				1,000	1,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Sep. 30, 2011
Net Income	(25,248)					(29,065)					14,462
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Dec. 31, 2011	$ 3,406,079					$ 2,848,977	$ 1				$ 974,542	$ 4	$ 1,111,262
Balance (in shares) at Dec. 31, 2011	235,999,750					1,000	1,000				1,000	1,000