CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
CASH FLOWS FROM (USED BY) OPERATING ACTIVITIES
Net Income	$ 81,612	$ 15,218
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depreciation, Depletion and Amortization, Nonproduction	187,178	172,708
Deferred Income Taxes and Tax Credits	34,425	7,889
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	(4,251)	(11,164)
Deferred energy	6,888	3,447
Amortization of other regulatory assets	75,711	78,080
Deferred rate increase	2,474	32,239
Other Noncash Income (Expense)	2,279	8,490
Changes in certain assets and liabilities:
Accounts receivable	(79,520)	(41,535)
Materials, supplies and fuel	(16,594)	(14,541)
Other current assets	1,518	4,961
Accounts payable	4,493	34,980
Accrued Retirement Benefits	4,907	6,755
Other current liabilities	(4,875)	(13,230)
Other deferred assets	(2,700)	(2,884)
Other regulatory assets	10,058	(61,796)
Other deferred liabilities	(10,940)	(17,098)
Net Cash from Operating Activities	292,663	202,519
CASH FLOWS USED BY INVESTING ACTIVITIES:
Additions to utility plant (excluding AFUDC-equity)	(261,559)	(310,207)
Proceeds from Sale of Other Property, Plant, and Equipment	0	131,789
Customer advances for construction	(847)	(3,789)
Contributions in aid of construction	45,106	47,213
Investments and other property - net	(128)	407
Net Cash used by Investing Activities	(217,428)	(134,587)
CASH FLOWS FROM (USED BY) FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	130,590	387,230
Retirement of long-term debt	(158,201)	(367,468)
Settlement of interest rate lock	0	(14,944)
Sale of Common Stock	0	8,384
Dividends paid	(70,800)	(56,587)
Net Cash from (used by) Financing Activities	(98,411)	(43,385)
Net Increase (Decrease) in Cash and Cash Equivalents	(23,176)	24,547
Beginning Balance in Cash and Cash Equivalents	145,944	86,189
Ending Balance in Cash and Cash Equivalents	122,768	110,736
Cash paid during period for:
Interest	148,603	156,656
Income taxes	1	1
Significant non-cash transactions:
Accrued Construction Expenses	121,785	115,412
NPC [Member]
CASH FLOWS FROM (USED BY) OPERATING ACTIVITIES
Net Income	60,981	7,043
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depreciation, Depletion and Amortization, Nonproduction	134,121	119,586
Deferred Income Taxes and Tax Credits	27,209	4,253
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	(2,990)	(9,953)
Deferred energy	13,127	18,088
Amortization of other regulatory assets	36,659	38,789
Deferred rate increase	2,474	32,239
Other Noncash Income (Expense)	(213)	1,474
Changes in certain assets and liabilities:
Accounts receivable	(101,041)	(62,992)
Materials, supplies and fuel	(4,167)	(153)
Other current assets	3,159	2,595
Accounts payable	41,388	47,878
Accrued Retirement Benefits	2,605	2,657
Other current liabilities	(3,342)	(7,412)
Other deferred assets	(1,973)	(2,405)
Other regulatory assets	24,720	(43,700)
Other deferred liabilities	(6,386)	(15,713)
Net Cash from Operating Activities	226,331	132,274
CASH FLOWS USED BY INVESTING ACTIVITIES:
Additions to utility plant (excluding AFUDC-equity)	(164,634)	(252,654)
Customer advances for construction	637	(1,660)
Contributions in aid of construction	25,383	38,229
Investments and other property - net	(144)	391
Net Cash used by Investing Activities	(138,758)	(215,694)
CASH FLOWS FROM (USED BY) FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	132,083	387,329
Retirement of long-term debt	(157,491)	(351,713)
Additional investment by parent company	0	54,000
Settlement of interest rate lock	0	(14,944)
Dividends paid	(79,000)	(25,000)
Net Cash from (used by) Financing Activities	(104,408)	49,672
Net Increase (Decrease) in Cash and Cash Equivalents	(16,835)	(33,748)
Beginning Balance in Cash and Cash Equivalents	65,887	60,077
Ending Balance in Cash and Cash Equivalents	49,052	26,329
Cash paid during period for:
Interest	105,973	110,916
Income taxes	1	1
Significant non-cash transactions:
Accrued Construction Expenses	98,358	97,905
SPPC [Member]
CASH FLOWS FROM (USED BY) OPERATING ACTIVITIES
Net Income	31,323	20,088
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Depreciation, Depletion and Amortization, Nonproduction	53,057	53,122
Deferred Income Taxes and Tax Credits	11,894	9,840
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	(1,261)	(1,211)
Deferred energy	(6,239)	(14,641)
Amortization of other regulatory assets	38,957	39,242
Other Noncash Income (Expense)	1,772	6,964
Changes in certain assets and liabilities:
Accounts receivable	21,543	21,518
Materials, supplies and fuel	(12,427)	(14,388)
Other current assets	(1,285)	4,304
Accounts payable	(27,583)	2,264
Accrued Retirement Benefits	936	3,796
Other current liabilities	(1,251)	(4,666)
Other deferred assets	(727)	(479)
Other regulatory assets	(14,662)	(18,096)
Other deferred liabilities	(3,173)	(2,468)
Net Cash from Operating Activities	90,874	105,189
CASH FLOWS USED BY INVESTING ACTIVITIES:
Additions to utility plant (excluding AFUDC-equity)	(96,925)	(57,553)
Proceeds from Sale of Other Property, Plant, and Equipment	0	131,789
Customer advances for construction	(1,484)	(2,129)
Contributions in aid of construction	19,723	8,984
Investments and other property - net	16	16
Net Cash used by Investing Activities	(78,670)	81,107
CASH FLOWS FROM (USED BY) FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt	(1,446)	0
Retirement of long-term debt	(710)	(15,755)
Dividends paid	(20,000)	(104,000)
Net Cash from (used by) Financing Activities	(22,156)	(119,755)
Net Increase (Decrease) in Cash and Cash Equivalents	(9,952)	66,541
Beginning Balance in Cash and Cash Equivalents	55,195	9,552
Ending Balance in Cash and Cash Equivalents	45,243	76,093
Cash paid during period for:
Interest	30,000	29,817
Income taxes	0	0
Significant non-cash transactions:
Accrued Construction Expenses	$ 23,427	$ 17,507