Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Consolidated quarterly financial information
Operating revenues	$ 699,762	$ 1,152,392	$ 915,822	$ 686,652	$ 693,122	$ 1,109,475	$ 878,576	$ 620,631	$ 3,454,628	$ 3,301,804	$ 3,241,379
Operations and maintenance	238,854	233,323	229,300	223,250	237,141	220,729	216,236	210,663	924,727	884,769	904,286
Operating income	83,900	415,688	259,812	86,923	101,289	447,970	254,489	48,007	846,323	851,755	746,508
Income Taxes	9,167	131,912	77,043	12,469	18,157	147,116	76,689	(4,645)	230,591	237,317	183,604
Income from continuing operations	32,814	234,718	139,598	32,836	34,905	252,874	130,930	284	439,966	418,993	355,634
Net income attributable to common shareholders	$ 24,260	$ 226,163	$ 131,207	$ 24,444	$ 22,631	$ 244,823	$ 122,345	$ (8,257)	$ 406,074	$ 381,542	$ 339,473
Earnings Per Share:
Income from continuing operations attributable to common shareholders - Basic (in dollars per share)	$ 0.22	$ 2.06	$ 1.19	$ 0.22	$ 0.24	$ 2.23	$ 1.12	$ (0.07)	$ 3.69	$ 3.54	$ 3.01
Net income attributable to common shareholders - Basic (in dollars per share)	$ 0.22	$ 2.06	$ 1.19	$ 0.22	$ 0.21	$ 2.23	$ 1.12	$ (0.08)	$ 3.69	$ 3.48	$ 3.11
Income from continuing operations attributable to common shareholders - Diluted (in dollars per share)	$ 0.22	$ 2.04	$ 1.18	$ 0.22	$ 0.24	$ 2.21	$ 1.12	$ (0.07)	$ 3.66	$ 3.50	$ 2.99
Net income attributable to common shareholders - Diluted (in dollars per share)	$ 0.22	$ 2.04	$ 1.18	$ 0.22	$ 0.20	$ 2.21	$ 1.11	$ (0.08)	$ 3.66	$ 3.45	$ 3.09