Consolidated Statement Of Net Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Consolidated Statement Of Net Income [Abstract]
Operating revenues	$ 198,204	$ 178,313	$ 368,448	$ 341,928
Operating expenses:
Operations and maintenance	66,980	66,765	135,245	130,496
Depreciation	28,696	26,550	56,291	52,826
Amortization	1,540	1,624	2,755	3,261
Taxes other than income taxes	12,593	10,751	22,683	21,802
Total expenses	109,809	105,690	216,974	208,385
Operating income	88,395	72,623	151,474	133,543
Other expense (income):
Interest expense, net	19,538	19,541	38,786	38,897
Allowance for funds used during construction	(1,244)	(1,929)	(2,588)	(3,906)
Gain on sale of other assets	(64)	(138)	(506)	(259)
Equity earnings in joint venture	(249)		(249)
Income from continuing operations before income taxes	70,414	55,149	116,031	98,811
Provision for income taxes	27,787	18,554	45,746	31,570
Income from continuing operations	42,627	36,595	70,285	67,241
Discontinued operations:
(Loss) income from discontinued operations before income taxes	(1,550)	1,652	15,367	1,130
Provision for income taxes	(368)	657	6,303	430
(Loss) Income from discontinued operations	(1,182)	995	9,064	700
Net income attributable to common shareholders	$ 41,445	$ 37,590	$ 79,349	$ 67,941
Income from continuing operations per share:
Basic	$ 0.31	$ 0.26	$ 0.51	$ 0.49
Diluted	$ 0.30	$ 0.26	$ 0.50	$ 0.49
Income from discontinued operations per share:
Basic	$ (0.01)	$ 0.01	$ 0.07	$ 0.01
Diluted	$ (0.01)	$ 0.01	$ 0.06	$ 0.01
Net income per common share:
Basic	$ 0.30	$ 0.27	$ 0.57	$ 0.49
Diluted	$ 0.30	$ 0.27	$ 0.57	$ 0.49
Average common shares outstanding during the period:
Basic	139,108	138,114	138,935	137,971
Diluted	139,843	138,781	139,577	138,518
Cash dividends declared per common share	$ 0.165	$ 0.155	$ 0.33	$ 0.31