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, 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	$ 1,111,262	$ (135,226)	$ (2,620)
Balance (in shares) at Dec. 31, 2010		235,322,553					1,000					1,000
Net Income	15,218			15,218		7,043			7,043		20,088			20,088
Dividend Reinvestment and Employee Benefits	8,072	654	7,418
Dividend Reinvestment and Employee Benefits (in shares)		654,489
Tax benefit from stock options exercised	312		312								312		312
Capital contribution from parent						54,000		54,000
Change in market value of risk management assets and liabilities	0
Change in compensation retirement benefits liability and amortization	1,873				1,873	953				953	1,951				1,951
Dividends Declared	(56,587)			(56,587)		(25,000)			(25,000)		(50,000)			(50,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Jun. 30, 2011	3,319,706	235,977	2,713,684	375,063	(5,018)	2,798,628	1	2,308,219	493,331	(2,923)	945,771	4	1,111,574	(165,138)	(669)
Balance (in shares) at Jun. 30, 2011		235,977,042					1,000					1,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Mar. 31, 2011
Net Income	12,888					16,063					3,512
Change in market value of risk management assets and liabilities	0
Change in compensation retirement benefits liability and amortization	731					200					390
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Jun. 30, 2011	3,319,706					2,798,628	1				945,771	4
Balance (in shares) at Jun. 30, 2011							1,000					1,000
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
Net Income	81,612			81,612		60,981			60,981		31,323			31,323
Change in market value of risk management assets and liabilities	(475)				(475)
Change in compensation retirement benefits liability and amortization	309				309	127				127	85				85
Dividends Declared	(70,800)			(70,800)		(79,000)			(79,000)		(20,000)			(20,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Jun. 30, 2012	3,416,725	236,000	2,713,736	475,089	(8,100)	2,831,085	1	2,308,219	526,855	(3,990)	985,950	4	1,111,262	(124,017)	(1,299)
Balance (in shares) at Jun. 30, 2012	235,999,750	235,999,750				1,000	1,000				1,000	1,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Mar. 31, 2012
Net Income	69,439					62,297					12,679
Change in market value of risk management assets and liabilities	(229)
Change in compensation retirement benefits liability and amortization	154					64					43
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Jun. 30, 2012	$ 3,416,725	$ 236,000	$ 2,713,736			$ 2,831,085	$ 1	$ 2,308,219			$ 985,950	$ 4	$ 1,111,262
Balance (in shares) at Jun. 30, 2012	235,999,750	235,999,750				1,000	1,000				1,000	1,000