Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				12 Months Ended			3 Months Ended				12 Months Ended		3 Months Ended				12 Months Ended		3 Months Ended				12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2010 As previously reported	Sep. 30, 2010 As previously reported	Jun. 30, 2010 As previously reported	Mar. 31, 2010 As previously reported	Dec. 31, 2010 As previously reported	Dec. 31, 2009 As previously reported	Dec. 31, 2010 Restatements	Sep. 30, 2010 Restatements	Jun. 30, 2010 Restatements	Mar. 31, 2010 Restatements	Dec. 31, 2010 Restatements	Dec. 31, 2009 Restatements	Dec. 31, 2010 Amount reported after reclassification	Sep. 30, 2010 Amount reported after reclassification	Jun. 30, 2010 Amount reported after reclassification	Mar. 31, 2010 Amount reported after reclassification	Dec. 31, 2010 Amount reported after reclassification	Dec. 31, 2009 Amount reported after reclassification
Operating Revenues
Other revenues					$ 4,185	$ 8,521	$ 4,469					$ 82,967	$ 26,723					$ (74,446)	$ (22,254)					$ 8,521	$ 4,469
Operating Expenses
Operations and maintenance	228,632	210,035	210,590	255,029	904,286	870,185	822,300	232,991	221,469	215,104	207,842	877,406	831,863	(2,386)	(1,811)	(1,495)	(1,529)	(7,221)	(9,563)	230,605	219,658	213,609	206,313	870,185	822,300
Depreciation and amortization					427,054	414,479	407,354					414,555	407,463					(76)	(109)					414,479	407,354
Taxes other than income taxes					147,408	135,328	123,270					135,334	123,277					(6)	(7)					135,328	123,270
Other expenses					6,659	7,509	5,984					65,651	24,534					(58,142)	(18,550)					7,509	5,984
Other
Other income					3,111	6,387	5,159					6,368	5,278					19	(119)					6,387	5,159
Other expense (Note 19)					(10,451)	(9,921)	(14,300)					(9,764)	(14,269)					(157)	(31)					(9,921)	(14,300)
Interest Expense
Interest charges					241,995	244,174	237,766						237,527						239						237,766
Allowance for borrowed funds used during construction					(18,358)	(16,479)	(10,379)					(16,539)	(10,430)					60	51					(16,479)	(10,379)
Income Taxes	7,375	131,416	50,818	(6,005)	183,604	160,869	138,551	(3,822)	123,486	51,829	(7,172)	164,321	136,506	(1,191)	(1,139)	(641)	(481)	(3,452)	2,045	(5,013)	122,347	51,188	(7,653)	160,869	138,551
Income From Continuing Operations	19,544	253,273	93,185	(10,368)	355,634	344,851	256,048	12,203	231,828	94,584	11,983	350,598	252,558	(1,919)	(2,266)	(1,270)	(292)	(5,747)	3,490	10,284	229,562	93,314	11,691	344,851	256,048
Income From Discontinued Operations					11,306	25,358	(183,284)					19,611	(179,794)					5,747	(3,490)					25,358	(183,284)
Balance Sheets
Current Assets - Other regulatory assets	69,072				69,072	62,286								62,286				62,286		62,286				62,286
Current Assets - Deferred income taxes	130,571				130,571	124,897		94,602				94,602		30,295				30,295		124,897				124,897
Deferred Debits - Regulatory assets	1,352,079				1,352,079	986,370		1,048,656				1,048,656		(62,286)				(62,286)		986,370				986,370
Current Liabilities - Deferred fuel and purchased power regulatory liability						58,442								58,442				58,442		58,442				58,442
Current Liabilities - Other regulatory liabilities	88,362				88,362	80,526								80,526				80,526		80,526				80,526
Deferred Credits and Other - Deferred income taxes	1,925,388				1,925,388	1,863,861		1,833,566				1,833,566		30,295				30,295		1,863,861				1,863,861
Deferred Credits and Other - Deferred fuel and purchased power regulatory liability								58,442				58,442		(58,442)				(58,442)
Deferred Credits and Other - Regulatory liabilities	737,332				737,332	614,063		694,589				694,589		(80,526)				(80,526)		614,063				614,063
Cash Flows from Operating Activities
Other current assets					(3,079)	9,375	38,406					5,246	24,647					4,129	13,759					9,375	38,406
Other current liabilities					20,358	2,921	57,280					5,204	29,274					(2,283)	28,006					2,921	57,280
Change in other regulatory liabilities				62,000	37,009	56,801	82,650					54,518	110,642					2,283	(27,992)					56,801	82,650
Change in other long-term assets					(41,722)	(47,940)	(64,629)					(43,189)	(47,899)					(4,751)	(16,730)					(47,940)	(64,629)
Expenditures for real estate investments												(622)	(2,957)					622	2,957
Other changes in real estate assets												4,068	(4,216)					(4,068)	4,216
Change in other long-term liabilities					$ 58,484	$ (97,388)	$ 12,161					$ (101,456)	$ 16,337					$ 4,068	$ (4,216)					$ (97,388)	$ 12,161