Consolidated Statements Of Income And Comprehensive Income (USD $) In Thousands, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Consolidated Statements Of Income And Comprehensive Income [Abstract]
Operating revenues	$ 197,328	$ 193,477	$ 539,256	$ 515,003
Operating expenses:
Operations and maintenance	70,039	68,908	200,535	198,448
Depreciation	27,132	26,494	79,884	77,635
Amortization	1,074	3,310	4,335	9,244
Taxes other than income taxes	11,324	11,442	33,126	32,161
Total expenses	109,569	110,154	317,880	317,488
Operating income	87,759	83,323	221,376	197,515
Other expense (income):
Interest expense, net	19,560	18,574	58,457	54,418
Allowance for funds used during construction	(1,804)	(1,018)	(5,710)	(3,895)
Gain on sale of other assets	(216)	(291)	(475)	(2,294)
Income from continuing operations before income taxes	70,219	66,058	169,104	149,286
Provision for income taxes	24,703	25,728	56,303	58,573
Income from continuing operations	45,516	40,330	112,801	90,713
Discontinued operations:
Income from discontinued operations before income taxes	5,138	5,747	6,194	7,385
Provision for income taxes	9,531	2,326	9,931	2,981
(Loss) income from discontinued operations	(4,393)	3,421	(3,737)	4,404
Net income attributable to common shareholders	41,123	43,751	109,064	95,117
Net income attributable to common shareholders	41,123	43,751	109,064	95,117
Other comprehensive income, net of tax:
Unrealized holding (loss) gain on investments	(373)	272	(277)	1,174
Reclassification adjustment for gains reported in net income	(83)		(156)	(1,330)
Comprehensive income	$ 40,667	$ 44,023	$ 108,631	$ 94,961
Income from continuing operations per share:
Basic	$ 0.33	$ 0.29	$ 0.82	$ 0.66
Diluted	$ 0.33	$ 0.29	$ 0.81	$ 0.66
(Loss) income from discontinued operations per share:
Basic	$ (0.03)	$ 0.02	$ (0.03)	$ 0.03
Diluted	$ (0.03)	$ 0.02	$ (0.03)	$ 0.03
Net income per common share:
Basic	$ 0.30	$ 0.32	$ 0.79	$ 0.70
Diluted	$ 0.30	$ 0.32	$ 0.79	$ 0.69
Average common shares outstanding during the period:
Basic	138,297	137,095	138,081	136,798
Diluted	138,951	137,394	138,625	137,112
Cash dividends declared per common share	$ 0.32	$ 0.30	$ 0.63	$ 0.59