Consolidation and Nature of Operations (Details) (USD $) In Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Operating Revenues
Other revenues	$ 792		$ 499	$ 2,795	$ 4,715
Operating Expenses
Operations and maintenance	210,035		219,658	675,654	639,580
Depreciation and amortization	106,350		104,177	319,550	307,806
Taxes other than income taxes	34,223		37,528	112,002	100,933
Other expenses	1,320		1,169	4,536	3,572
Other
Other income	441		4,261	2,630	3,851
Other expense (Note 11)	(3,052)		(3,894)	(7,921)	(8,768)
Interest Expense
Interest charges	62,034		60,419	183,251	181,937
Allowance for borrowed funds used during construction	(6,939)		(6,163)	(14,371)	(12,254)
Income Taxes	131,416		122,347	176,229	165,882
Income From Continuing Operations	253,273		229,562	336,090	334,567
Income From Discontinued Operations	9,512		9,477	10,860	23,141
Balance Sheets
Current Assets - Deferred income taxes	107,732			107,732		124,897
Current Assets - Other regulatory assets	55,852			55,852		62,286
Deferred Debits - Regulatory assets	977,975			977,975		986,370
Current Liabilities - Deferred fuel and purchased power regulatory liability						58,442
Current Liabilities - Other regulatory liabilities	94,374			94,374		80,526
Deferred Credits and Other - Deferred income taxes	1,955,458			1,955,458		1,863,861
Deferred Credits and Other - Regulatory liabilities	689,120			689,120		614,063
Cash Flows from Operating Activities
Other current assets				(1,581)	(6,027)
Other current liabilities				30,300	6,738
Change in other regulatory liabilities		53,000
Change in other long-term assets				(10,678)	(59,382)
Change in other long-term liabilities				74,565	17,636
Reclassifications for discontinued operations
Operating Revenues
Other revenues			(22,375)		(48,267)
Operating Expenses
Operations and maintenance			(1,811)		(4,835)
Depreciation and amortization			(17)		(58)
Taxes other than income taxes					(3)
Other expenses			(17,196)		(37,437)
Other
Other income			(87)		23
Other expense (Note 11)			(39)		(118)
Interest Expense
Interest charges			(72)
Allowance for borrowed funds used during construction					60
Income Taxes			(1,139)		(2,261)
Income From Continuing Operations			(2,266)		(3,828)
Income From Discontinued Operations			2,266		3,828
Reclassifications for regulatory assets and liabilities
Balance Sheets
Current Assets - Deferred income taxes						30,295
Current Assets - Other regulatory assets						62,286
Deferred Debits - Regulatory assets						(62,286)
Current Liabilities - Deferred fuel and purchased power regulatory liability						58,442
Current Liabilities - Other regulatory liabilities						80,526
Deferred Credits and Other - Deferred income taxes						30,295
Deferred Credits and Other - Deferred fuel and purchased power regulatory liability						(58,442)
Deferred Credits and Other - Regulatory liabilities						(80,526)
Cash Flows from Operating Activities
Other current assets					7,209
Other current liabilities					(4,251)
Expenditures for real estate investments					514
Gains and other changes in real estate assets					(1,811)
Change in other regulatory liabilities					(40,121)
Change in other long-term assets					(7,723)
Change in other long-term liabilities					46,183
As previously reported
Operating Revenues
Other revenues			22,874		52,982
Operating Expenses
Operations and maintenance			221,469		644,415
Depreciation and amortization			104,194		307,864
Taxes other than income taxes					100,936
Other expenses			18,365		41,009
Other
Other income			4,348		3,828
Other expense (Note 11)			(3,855)		(8,650)
Interest Expense
Interest charges			60,491
Allowance for borrowed funds used during construction					(12,314)
Income Taxes			123,486		168,143
Income From Continuing Operations			231,828		338,395
Income From Discontinued Operations			7,211		19,313
Balance Sheets
Current Assets - Deferred income taxes						94,602
Deferred Debits - Regulatory assets						1,048,656
Deferred Credits and Other - Deferred income taxes						1,833,566
Deferred Credits and Other - Deferred fuel and purchased power regulatory liability						58,442
Deferred Credits and Other - Regulatory liabilities						694,589
Cash Flows from Operating Activities
Other current assets					(13,236)
Other current liabilities					10,989
Expenditures for real estate investments					(514)
Gains and other changes in real estate assets					1,811
Change in other regulatory liabilities					40,121
Change in other long-term assets					(51,659)
Change in other long-term liabilities					(28,547)
Amount reported after reclassification
Operating Revenues
Other revenues			499		4,715
Operating Expenses
Operations and maintenance			219,658		639,580
Depreciation and amortization			104,177		307,806
Taxes other than income taxes					100,933
Other expenses			1,169		3,572
Other
Other income			4,261		3,851
Other expense (Note 11)			(3,894)		(8,768)
Interest Expense
Interest charges			60,419
Allowance for borrowed funds used during construction					(12,254)
Income Taxes			122,347		165,882
Income From Continuing Operations			229,562		334,567
Income From Discontinued Operations			9,477		23,141
Balance Sheets
Current Assets - Deferred income taxes						124,897
Current Assets - Other regulatory assets						62,286
Deferred Debits - Regulatory assets						986,370
Current Liabilities - Deferred fuel and purchased power regulatory liability						58,442
Current Liabilities - Other regulatory liabilities						80,526
Deferred Credits and Other - Deferred income taxes						1,863,861
Deferred Credits and Other - Regulatory liabilities						614,063
Cash Flows from Operating Activities
Other current assets					(6,027)
Other current liabilities					6,738
Change in other long-term assets					(59,382)
Change in other long-term liabilities					$ 17,636