CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
CASH FLOWS FROM (USED BY) OPERATING ACTIVITIES
Net Income	$ 12,173	$ 2,330
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depreciation, Depletion and Amortization, Nonproduction	90,862	83,102
Deferred Income Taxes and Tax Credits	(5,183)	(414)
AFUDC - equity	1,932	7,642
Deferred energy	(9,134)	3,118
Gain (Loss) on Sale of Investments	0	0
Amortization of other regulatory assets	39,028	38,839
Deferred rate increase	(2,691)	(15,334)
Other Noncash Income (Expense)	1,575	3,269
Changes in certain assets and liabilities:
Accounts receivable	31,208	38,185
Materials, supplies and fuel	(874)	(8,639)
Other current assets	(13,021)	(5,653)
Accounts payable	(37,825)	(23,364)
Accrued Retirement Benefits	2,221	3,521
Other current liabilities	(29,348)	(34,708)
Other deferred assets	(1,602)	(511)
Other regulatory assets	4,164	(18,608)
Other deferred liabilities	(17,687)	(2,636)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	67,316	85,523
CASH FLOWS USED BY INVESTING ACTIVITIES:
Additions to utility plant (excluding AFUDC-equity)	(115,817)	(144,097)
Proceeds from Sale of Other Property, Plant, and Equipment	0	131,789
Customer advances for construction	(184)	(672)
Contributions in aid of construction	26,052	20,178
Investments and other property - net	48	286
Net Cash Provided by (Used in) Investing Activities	(89,901)	7,484
CASH FLOWS FROM (USED BY) FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	10,951	0
Retirement of long-term debt	(3,295)	(18,481)
Settlement of interest rate lock	0	0
Sale of Common Stock	0	5,353
Dividends paid	(30,680)	(28,276)
Net Cash from (used by) Financing Activities	(23,024)	(41,404)
Net Increase (Decrease) in Cash and Cash Equivalents	(45,609)	51,603
Beginning Balance in Cash and Cash Equivalents	145,944	86,189
Ending Balance in Cash and Cash Equivalents	100,335	137,792
Cash paid during period for:
Interest	88,606	92,323
Income taxes	0	1
Significant non-cash transactions:
Accrued Construction Expenses	85,850	77,033
Capital Lease Obligations Incurred	0	0
NPC [Member]
CASH FLOWS FROM (USED BY) OPERATING ACTIVITIES
Net Income	(1,316)	(9,020)
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depreciation, Depletion and Amortization, Nonproduction	64,990	57,673
Deferred Income Taxes and Tax Credits	(6,349)	(4,725)
AFUDC - equity	1,413	7,098
Deferred energy	4,050	9,980
Amortization of other regulatory assets	18,301	18,870
Deferred rate increase	2,691	15,334
Other Noncash Income (Expense)	(796)	2,288
Changes in certain assets and liabilities:
Accounts receivable	22,441	35,397
Materials, supplies and fuel	(2,209)	(2,971)
Other current assets	(8,674)	(6,075)
Accounts payable	(14,248)	(15,946)
Accrued Retirement Benefits	1,572	1,313
Other current liabilities	(27,419)	(25,513)
Other deferred assets	(1,288)	(781)
Other regulatory assets	9,880	(12,328)
Other deferred liabilities	(7,495)	(1,911)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	52,718	54,487
CASH FLOWS USED BY INVESTING ACTIVITIES:
Additions to utility plant (excluding AFUDC-equity)	(66,843)	(120,455)
Proceeds from Sale of Other Property, Plant, and Equipment	0	0
Customer advances for construction	654	(211)
Contributions in aid of construction	15,951	16,479
Investments and other property - net	40	278
Net Cash Provided by (Used in) Investing Activities	(50,198)	(103,909)
CASH FLOWS FROM (USED BY) FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	12,432	0
Retirement of long-term debt	(3,129)	(3,007)
Additional investment by parent company	0	54,000
Settlement of interest rate lock	0	0
Dividends paid	(39,000)	0
Net Cash from (used by) Financing Activities	(29,697)	50,993
Net Increase (Decrease) in Cash and Cash Equivalents	(27,177)	1,571
Beginning Balance in Cash and Cash Equivalents	65,887	60,077
Ending Balance in Cash and Cash Equivalents	38,710	61,648
Cash paid during period for:
Interest	71,276	69,936
Income taxes	0	1
Significant non-cash transactions:
Accrued Construction Expenses	72,179	66,894
Capital Lease Obligations Incurred	0	0
SPPC [Member]
CASH FLOWS FROM (USED BY) OPERATING ACTIVITIES
Net Income	18,644	16,576
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depreciation, Depletion and Amortization, Nonproduction	25,872	25,429
Deferred Income Taxes and Tax Credits	3,537	8,109
AFUDC - equity	519	544
Deferred energy	(13,184)	(6,862)
Gain (Loss) on Sale of Investments	0	0
Amortization of other regulatory assets	20,668	19,944
Other Noncash Income (Expense)	2,249	1,136
Changes in certain assets and liabilities:
Accounts receivable	8,869	2,837
Materials, supplies and fuel	1,335	(5,668)
Other current assets	(4,564)	(479)
Accounts payable	(17,675)	4,835
Accrued Retirement Benefits	367	1,703
Other current liabilities	(5,388)	(8,667)
Other deferred assets	(314)	270
Other regulatory assets	(5,716)	(6,280)
Other deferred liabilities	(4,214)	(104)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	29,967	52,235
CASH FLOWS USED BY INVESTING ACTIVITIES:
Additions to utility plant (excluding AFUDC-equity)	(48,974)	(23,642)
Proceeds from Sale of Other Property, Plant, and Equipment	0	131,789
Customer advances for construction	(838)	(461)
Contributions in aid of construction	10,101	3,699
Investments and other property - net	8	8
Net Cash Provided by (Used in) Investing Activities	(39,703)	111,393
CASH FLOWS FROM (USED BY) FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	(1,441)	0
Retirement of long-term debt	(166)	(15,390)
Additional investment by parent company	0	0
Dividends paid	(20,000)	(92,000)
Net Cash from (used by) Financing Activities	(21,607)	(107,390)
Net Increase (Decrease) in Cash and Cash Equivalents	(31,343)	56,238
Beginning Balance in Cash and Cash Equivalents	55,195	9,552
Ending Balance in Cash and Cash Equivalents	23,852	65,790
Cash paid during period for:
Interest	15,944	15,925
Income taxes	0	0
Significant non-cash transactions:
Accrued Construction Expenses	13,671	10,139
Transfer of assets to accounts receivable	$ 0	$ 0