CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
CASH FLOWS FROM OPERATING ACTIVITIES:
Net Income	$ 163,432	$ 226,984	$ 182,936
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depreciation, Depletion and Amortization, Nonproduction	357,937	333,059	321,921
Deferred Income Taxes and Tax Credits	88,445	129,231	111,219
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	(10,873)	(28,112)	(24,274)
Deferred energy	(55,429)	147,497	306,406
Gain (Loss) on Sale of Investments	0	(7,575)	0
Amortization of other regulatory assets	166,095	110,654	101,641
Deferred rate increase	79,866	(8,343)	(95,890)
Other Noncash Income (Expense)	16,536	(20,666)	(7,755)
Changes in certain assets and liabilities:
Accounts receivable	215	52,238	12,733
Materials, supplies and fuel	(14,747)	9,167	465
Other current assets	5,548	1,969	8,335
Accounts payable	17,466	28,070	(31,888)
Increase (Decrease) in Pension and Postretirement Obligations	(26,845)	(18,476)	(20,080)
Other current liabilities	(14,449)	2,945	(17,287)
Increase (Decrease) in Derivative Assets and Liabilities	3,810	12,267	5,058
Other deferred assets	(6,430)	(6,111)	(13,831)
Other regulatory assets	(113,568)	(77,893)	(69,937)
Other deferred liabilities	1,369	(453)	(18,251)
Net Cash from Operating Activities	658,378	886,452	751,521
CASH FLOWS USED BY INVESTING ACTIVITIES:
Additions to utility plant (excluding AFUDC-equity)	(620,516)	(629,496)	(843,132)
Proceeds from Sale of Other Property, Plant, and Equipment	166,603	18,225	0
Customer advances for construction	(7,762)	(11,142)	(8,369)
Contributions in aid of construction	106,050	63,330	76,940
Investments and other property - net	498	(8,974)	(26,061)
Net Cash used by Investing Activities	(355,127)	(568,057)	(800,622)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	579,820	985,419	1,418,872
Retirement of long-term debt	(701,244)	(1,180,646)	(1,271,350)
Settlement of interest rate lock	(14,944)	0	0
Sale of Common Stock	8,459	6,114	6,051
Dividends paid	(115,587)	(105,799)	(96,125)
Net Cash from/(used by) Financing Activities	(243,496)	(294,912)	57,448
Net Increase in Cash and Cash Equivalents	59,755	23,483	8,347
Beginning Balance in Cash and Cash Equivalents	86,189	62,706	54,359
Ending Balance in Cash and Cash Equivalents	145,944	86,189	62,706
Cash paid during period for:
Interest	314,401	336,668	325,508
Income taxes	576	754	(13,186)
Noncash Investing and Financing Items [Abstract]
Noncash or Part Noncash Acquisition, Fixed Assets Acquired	195,511	86,127	127,786
Capital Lease Obligations Incurred	0	15,336	0
Transfer of assets to accounts receivable	0	16,830	0
NPC [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net Income	132,586	185,943	134,284
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depreciation, Depletion and Amortization, Nonproduction	252,191	226,252	215,873
Deferred Income Taxes and Tax Credits	71,971	92,859	96,831
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	(8,298)	(25,229)	(21,025)
Deferred energy	3,549	116,230	216,629
Amortization of other regulatory assets	83,070	74,625	61,758
Deferred rate increase	79,866	(8,343)	(95,890)
Other Noncash Income (Expense)	7,147	(16,153)	(159)
Changes in certain assets and liabilities:
Accounts receivable	(8,391)	39,679	(5,309)
Materials, supplies and fuel	(5,674)	3,115	4,928
Other current assets	736	(1,824)	6,802
Accounts payable	(11)	13,905	(10,694)
Increase (Decrease) in Pension and Postretirement Obligations	(9,725)	(17,792)	(18,721)
Other current liabilities	(7,888)	4,959	(13,544)
Increase (Decrease) in Derivative Assets and Liabilities	2,225	9,565	3,319
Other deferred assets	(5,125)	(2,598)	(10,336)
Other regulatory assets	(54,885)	(50,937)	(54,061)
Other deferred liabilities	(6,235)	(2,873)	(25,611)
Net Cash from Operating Activities	527,109	641,383	485,074
CASH FLOWS USED BY INVESTING ACTIVITIES:
Additions to utility plant (excluding AFUDC-equity)	(475,118)	(499,374)	(656,074)
Proceeds from Sale of Other Property, Plant, and Equipment	31,997	3,254	0
Customer advances for construction	(1,852)	(8,646)	(5,281)
Contributions in aid of construction	89,427	55,140	67,514
Investments and other property - net	475	(5)	(21,547)
Net Cash used by Investing Activities	(355,071)	(449,631)	(615,388)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	386,884	637,463	1,065,338
Retirement of long-term debt	(493,168)	(737,747)	(809,009)
Additional investment by parent company	54,000	0	0
Settlement of interest rate lock	(14,944)	0	0
Dividends paid	(99,000)	(74,000)	(112,000)
Net Cash from/(used by) Financing Activities	(166,228)	(174,284)	144,329
Net Increase in Cash and Cash Equivalents	5,810	17,468	14,015
Beginning Balance in Cash and Cash Equivalents	60,077	42,609	28,594
Ending Balance in Cash and Cash Equivalents	65,887	60,077	42,609
Cash paid during period for:
Interest	218,693	226,138	217,807
Income taxes	1	2	2
Noncash Investing and Financing Items [Abstract]
Noncash or Part Noncash Acquisition, Fixed Assets Acquired	175,661	74,557	117,226
Capital Lease Obligations Incurred	0	15,336	0
SPPC [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net Income	59,886	72,375	73,085
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depreciation, Depletion and Amortization, Nonproduction	105,746	106,807	106,048
Deferred Income Taxes and Tax Credits	31,487	39,220	32,548
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	(2,575)	(2,883)	(3,249)
Deferred energy	(58,978)	31,267	89,777
Gain (Loss) on Sale of Investments	0	(7,575)	0
Amortization of other regulatory assets	81,636	35,799	39,146
Other Noncash Income (Expense)	8,464	(7,929)	(8,778)
Changes in certain assets and liabilities:
Accounts receivable	8,739	31,961	68,435
Materials, supplies and fuel	(9,073)	5,991	(4,436)
Other current assets	4,128	4,421	1,575
Accounts payable	26,564	2,050	(15,071)
Increase (Decrease) in Pension and Postretirement Obligations	(18,401)	(2,523)	(2,227)
Other current liabilities	(2,131)	721	(3,038)
Increase (Decrease) in Derivative Assets and Liabilities	531	2,702	1,739
Other deferred assets	(1,305)	(3,513)	(3,495)
Other regulatory assets	(58,683)	(26,956)	(15,876)
Other deferred liabilities	641	887	(30,388)
Net Cash from Operating Activities	176,676	282,822	325,795
CASH FLOWS USED BY INVESTING ACTIVITIES:
Additions to utility plant (excluding AFUDC-equity)	(145,398)	(143,216)	(187,058)
Proceeds from Sale of Other Property, Plant, and Equipment	134,606	14,971	0
Customer advances for construction	(5,910)	(2,496)	(3,088)
Contributions in aid of construction	16,623	8,190	9,426
Investments and other property - net	23	(97)	(5,017)
Net Cash used by Investing Activities	(56)	(122,648)	(185,737)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	(403)	37,726	353,534
Retirement of long-term debt	(16,574)	(148,707)	(462,144)
Additional investment by parent company	0	0	90,300
Dividends paid	(114,000)	(54,000)	(128,800)
Net Cash from/(used by) Financing Activities	(130,977)	(164,981)	(147,110)
Net Increase in Cash and Cash Equivalents	45,643	(4,807)	(7,052)
Beginning Balance in Cash and Cash Equivalents	9,552	14,359	21,411
Ending Balance in Cash and Cash Equivalents	55,195	9,552	14,359
Cash paid during period for:
Interest	59,605	67,351	69,966
Income taxes	575	752	12
Noncash Investing and Financing Items [Abstract]
Noncash or Part Noncash Acquisition, Fixed Assets Acquired	19,850	11,570	10,560
Transfer of assets to accounts receivable	0	16,830	0
Dividends	$ 0	$ 54,000	$ 0