Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, 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	Mar. 31, 2011 Restatements	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
Consolidated quarterly financial information
Operating revenues	$ 667,892	$ 1,124,841	$ 799,799	$ 648,847	$ 3,241,379	$ 3,189,199	$ 3,153,656	$ 683,611	$ 1,139,085	$ 820,594	$ 620,355	$ 3,263,645			$ (26,179)	$ (22,375)	$ (17,799)	$ (8,093)	$ (74,446)		$ 657,432	$ 1,116,710	$ 802,795	$ 612,262	$ 3,189,199
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
Operating income	78,715	435,017	196,992	35,784	746,508	714,883	616,128	59,318	403,625	203,273	57,668	723,884			(3,067)	(3,351)	(1,889)	(694)	(9,001)		56,251	400,274	201,384	56,974	714,883
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 (loss) 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
Net income (loss) attributable to common shareholders	12,564	255,359	86,685	(15,135)	339,473	350,053	68,330	7,350	233,920	114,797	(6,014)	350,053									7,350	233,920	114,797	(6,014)	350,053
Earnings Per Share:
Income (loss) from continuing operations attributable to common shareholders - Basic (in dollars per share)	$ 0.11	$ 2.25	$ 0.79	$ (0.15)	$ 3.01	$ 3.05	$ 2.34	$ 0.07	$ 2.09	$ 0.84	$ 0.07	$ 3.10									$ 0.05	$ 2.07	$ 0.82	$ 0.06	$ 3.05
Net income (loss) attributable to common shareholders - Basic (in dollars per share)	$ 0.12	$ 2.34	$ 0.80	$ (0.14)	$ 3.11	$ 3.28	$ 0.68	$ 0.07	$ 2.15	$ 1.07	$ (0.06)	$ 3.28									$ 0.07	$ 2.15	$ 1.07	$ (0.06)	$ 3.28
Income (loss) from continuing operations attributable to common shareholders - Diluted (in dollars per share)	$ 0.11	$ 2.24	$ 0.78	$ (0.15)	$ 2.99	$ 3.03	$ 2.34	$ 0.06	$ 2.08	$ 0.83	$ 0.07	$ 3.08									$ 0.05	$ 2.06	$ 0.82	$ 0.06	$ 3.03
Net income (loss) attributable to common shareholders - Diluted (in dollars per share)	$ 0.11	$ 2.32	$ 0.79	$ (0.14)	$ 3.09	$ 3.27	$ 0.67	$ 0.07	$ 2.14	$ 1.07	$ (0.06)	$ 3.27									$ 0.07	$ 2.14	$ 1.07	$ (0.06)	$ 3.27
Reduction in operating revenues														10,728
Reduction in operations and maintenance														1,457
Reduction in operating income														1,357
Decrease in income taxes														356
Decrease in income from continuing operations														$ 1,043