Consolidated Statements Of Net Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Consolidated Statements Of Net Income [Abstract]
Operating revenues	$ 204,345	$ 214,565	$ 580,035	$ 570,279
Operating expenses:
Operations and maintenance	72,065	71,268	211,234	199,664
Depreciation	30,188	28,251	88,971	82,736
Amortization	1,175	1,320	3,903	3,773
Taxes other than income taxes	13,537	13,191	40,321	34,700
Total expenses	116,965	114,030	344,429	320,873
Operating income	87,380	100,535	235,606	249,406
Other expense (income):
Interest expense, net	19,350	19,597	57,834	58,384
Allowance for funds used during construction	(426)	(919)	(1,468)	(3,484)
Gain on sale of other assets	(138)	(320)	(121)	(826)
Equity loss (earnings) in joint venture	(78)	(682)	1,732	(931)
Income from continuing operations before income taxes	68,672	82,859	177,629	196,263
Provision for income taxes	5,188	32,575	19,366	77,310
Income from continuing operations	63,484	50,284	158,263	118,953
Discontinued operations:
Income from discontinued operations before income taxes	193	819	8,524	18,813
Provision for income taxes	60	444	3,019	7,758
Income from discontinued operations	133	375	5,505	11,055
Net income attributable to common shareholders	$ 63,617	$ 50,659	$ 163,768	$ 130,008
Income from continuing operations per share:
Basic	$ 0.36	$ 0.29	$ 0.90	$ 0.68
Diluted	$ 0.36	$ 0.29	$ 0.90	$ 0.68
Income from discontinued operations per share:
Basic	$ 0.00	$ 0.00	$ 0.03	$ 0.06
Diluted	$ 0.00	$ 0.00	$ 0.03	$ 0.06
Net income per common share:
Basic	$ 0.36	$ 0.29	$ 0.93	$ 0.75
Diluted	$ 0.36	$ 0.29	$ 0.93	$ 0.74
Average common shares outstanding during the period:
Basic	176,483	174,596	175,964	173,981
Diluted	177,575	175,608	176,732	174,688
Cash dividends declared per common share		$ 0.272	$ 0.432	$ 0.536