Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated quarterly financial information
Operating revenues	$ 693,122	$ 1,109,475	$ 878,576	$ 620,631	$ 667,892	$ 1,124,841	$ 799,799	$ 648,847	$ 3,301,804	$ 3,241,379	$ 3,189,199
Operations and maintenance	237,141	220,729	216,236	210,663	228,632	210,035	210,590	255,029	884,769	904,286	870,185
Operating income	101,289	447,970	254,489	48,007	78,715	435,017	196,992	35,784	851,755	746,508	714,883
Income Taxes	18,157	147,116	76,689	(4,645)	7,375	131,416	50,818	(6,005)	237,317	183,604	160,869
Income from continuing operations	34,905	252,874	130,930	284	19,544	253,273	93,185	(10,368)	418,993	355,634	344,851
Net income (loss) attributable to common shareholders	$ 22,631	$ 244,823	$ 122,345	$ (8,257)	$ 12,564	$ 255,359	$ 86,685	$ (15,135)	$ 381,542	$ 339,473	$ 350,053
Earnings Per Share:
Income (loss) from continuing operations attributable to common shareholders - Basic (in dollars per share)	$ 0.24	$ 2.23	$ 1.12	$ (0.07)	$ 0.11	$ 2.25	$ 0.79	$ (0.15)	$ 3.54	$ 3.01	$ 3.05
Net income (loss) attributable to common shareholders - Basic (in dollars per share)	$ 0.21	$ 2.23	$ 1.12	$ (0.08)	$ 0.12	$ 2.34	$ 0.80	$ (0.14)	$ 3.48	$ 3.11	$ 3.28
Income (loss) from continuing operations attributable to common shareholders - Diluted (in dollars per share)	$ 0.24	$ 2.21	$ 1.12	$ (0.07)	$ 0.11	$ 2.24	$ 0.78	$ (0.15)	$ 3.50	$ 2.99	$ 3.03
Net income (loss) attributable to common shareholders - Diluted (in dollars per share)	$ 0.20	$ 2.21	$ 1.11	$ (0.08)	$ 0.11	$ 2.32	$ 0.79	$ (0.14)	$ 3.45	$ 3.09	$ 3.27