CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
OPERATING REVENUES:
OPERATING REVENUES	$ 2,943,307		$ 3,280,222		$ 3,585,798
Operating Expenses
Fuel for power generation	680,585		821,484		881,768
Purchased power	633,874		648,881		758,736
Gas purchased for resale	125,155		137,702		153,607
Deferred energy	(83,333)		113,107		289,076
Energy efficiency program costs	43,537		0		0
Other operating expenses	411,115		414,241		453,413
Maintenance	103,307		104,567		102,309
Depreciation and amortization	357,937		333,059		321,921
Taxes other than income	60,465		62,746		60,885
Total Operating Expenses	2,332,642		2,635,787		3,021,715
OPERATING INCOME	610,665		644,435		564,083
OTHER INCOME (EXPENSE):
Interest expense (net of AFUDC-debt)	(328,710)		(333,010)		(334,314)
Interest income (expense) on regulatory items	(16,321)		(12,517)		(2,280)
AFUDC - equity	10,873		28,112		24,274
Other income	22,764		36,841		33,122
Other expense	(48,924)		(23,113)		(26,498)
Total Other Income (Expense)	(360,318)		(303,687)		(305,696)
Income Before Income Tax Expense	250,347		340,748		258,387
Income Tax Expense (Benefit)	86,915		113,764		75,451
NET INCOME	163,432		226,984		182,936
Other comprehensive income (loss):
Change in compensation retirement benefits liability and amortization	(357)		(403)		(128)
Change in market value of risk management assets and liabilities	(686)		0		0
OTHER COMPREHENSIVE INCOME(LOSS)	(1,043)		(403)		(128)
COMPREHENSIVE INCOME	162,389		226,581		182,808
Amount per share basic and diluted - (Note 15)
Net income per share - basic	$ 0.69		$ 0.97		$ 0.78
Net income per share - diluted	$ 0.69		$ 0.96		$ 0.78
Weighted Average Shares of Common Stock Outstanding - basic	235,847,596		235,048,347		234,542,292
Weighted Average Shares of Common Stock Outstanding - diluted	237,767,071	[1]	236,294,812	[1]	235,180,688	[1]
Dividends Declared Per Share of Common Stock	$ 0.49		$ 0.45		$ 0.41
NPC [Member]
OPERATING REVENUES:
OPERATING REVENUES	2,054,393		2,252,377		2,423,377
Operating Expenses
Fuel for power generation	498,487		588,419		587,647
Purchased power	477,226		505,239		627,759
Deferred energy	(16,300)		94,843		207,611
Energy efficiency program costs	37,292
Other operating expenses	260,127		260,535		279,865
Maintenance	64,320		71,759		71,019
Depreciation and amortization	252,191		226,252		215,873
Taxes other than income	37,254		37,918		37,241
Total Operating Expenses	1,610,597		1,784,965		2,027,015
OPERATING INCOME	443,796		467,412		396,362
OTHER INCOME (EXPENSE):
Interest expense (net of AFUDC-debt)	(221,953)		(214,367)		(226,252)
Interest income (expense) on regulatory items	(8,572)		(3,169)		3,463
AFUDC - equity	8,298		25,229		21,025
Other income	14,774		15,541		19,658
Other expense	(33,020)		(12,946)		(18,320)
Total Other Income (Expense)	(240,473)		(189,712)		(200,426)
Income Before Income Tax Expense	203,323		277,700		195,936
Income Tax Expense (Benefit)	70,737		91,757		61,652
NET INCOME	132,586		185,943		134,284
Other comprehensive income (loss):
Change in compensation retirement benefits liability and amortization	(241)		(380)		175
COMPREHENSIVE INCOME	132,345		185,563		134,459
SPPC [Member]
OPERATING REVENUES:
Electric	716,417		836,879		957,130
Gas	172,482		190,943		205,263
OPERATING REVENUES	888,899		1,027,822		1,162,393
Operating Expenses
Fuel for power generation	182,098		233,065		294,121
Purchased power	156,648		143,642		130,977
Gas purchased for resale	125,155		137,702		153,607
Deferral of energy - electric-net	(65,445)		8,475		73,829
Deferral of energy - gas-net	(1,588)		9,789		7,636
Energy efficiency program costs	6,245
Other operating expenses	146,699		149,946		170,849
Maintenance	38,987		32,808		31,290
Depreciation and amortization	105,746		106,807		106,048
Taxes other than income	22,921		24,593		23,447
Total Operating Expenses	717,466		846,827		991,804
OPERATING INCOME	171,433		180,995		170,589
OTHER INCOME (EXPENSE):
Interest expense (net of AFUDC-debt)	(67,435)		(68,514)		(69,413)
Interest income (expense) on regulatory items	(7,749)		(9,348)		(5,743)
AFUDC - equity	2,575		2,883		3,249
Other income	6,883		16,748		13,276
Other expense	(14,624)		(9,985)		(7,648)
Total Other Income (Expense)	(80,350)		(68,216)		(66,279)
Income Before Income Tax Expense	91,083		112,779		104,310
Income Tax Expense (Benefit)	31,197		40,404		31,225
NET INCOME	59,886		72,375		73,085
Other comprehensive income (loss):
Change in compensation retirement benefits liability and amortization	1,236		(215)		(87)
COMPREHENSIVE INCOME	$ 61,122		$ 72,160		$ 72,998

[1] The denominator does not include stock equivalents for options issued under the LTIP due to conversion prices being higher than market prices for all periods. Under this plan, an additional 557,793, 701,658 and 679,272 shares for 2011, 2010 and 2009, respectively, would be included in each of these periods if the conditions for conversion were met.