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	2,330			2,330		(9,020)			(9,020)		16,576			16,576
Dividend Reinvestment and Employee Benefits	5,041	450	4,591
Dividend Reinvestment and Employee Benefits (in shares)		450,852
Tax benefit from stock options exercised	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	1,142				1,142	753				753	1,561				1,561
Dividends Declared	(28,276)			(28,276)							(38,000)			(38,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Mar. 31, 2011	3,331,367	235,773	2,710,857	390,486	(5,749)	2,807,365	1	2,308,219	502,268	(3,123)	953,869	4	1,111,574	(156,650)	(1,059)
Balance (in shares) at Mar. 31, 2011		235,773,405					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	12,173			12,173		(1,316)			(1,316)		18,644			18,644
Change in market value of risk management assets and liabilities	(246)				(246)
Change in compensation retirement benefits liability and amortization	155				155	63				63	42				42
Dividends Declared	(30,680)			(30,680)		(39,000)			(39,000)		(20,000)			(20,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest at Mar. 31, 2012	$ 3,387,481	$ 236,000	$ 2,713,736	$ 445,770	$ (8,025)	$ 2,808,724	$ 1	$ 2,308,219	$ 504,558	$ (4,054)	$ 973,228	$ 4	$ 1,111,262	$ (136,696)	$ (1,342)
Balance (in shares) at Mar. 31, 2012	235,999,750	235,999,750				1,000	1,000				1,000	1,000