Consolidated Statements Of Net Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Consolidated Statements Of Net Income [Abstract]
Operating revenues	$ 210,535	$ 202,320	$ 588,514	$ 574,815
Operating expenses:
Operations and maintenance	72,374	71,631	214,435	209,837
Depreciation	29,482	29,980	91,689	88,336
Amortization	806	1,185	2,685	3,930
Taxes other than income taxes	12,815	13,384	37,943	39,884
Total operating expenses	115,477	116,180	346,752	341,987
Operating income	95,058	86,140	241,762	232,828
Other expense (income):
Interest expense, net	18,990	19,350	57,393	57,833
Allowance for funds used during construction	(1,195)	(426)	(3,299)	(1,468)
Loss (gain) on sale of other assets	(75)	(138)	133	(121)
Equity loss (earnings) in joint venture	736	(78)	2,673	1,732
Income from continuing operations before income taxes	76,602	67,432	184,862	174,852
Provision for income taxes	8,891	4,683	19,932	18,235
Income from continuing operations	67,711	62,749	164,930	156,617
Discontinued operations:
Income from discontinued operations before income taxes	472	1,433	2,497	11,301
Provision for income taxes	187	565	1,003	4,150
Income from discontinued operations	285	868	1,494	7,151
Net income attributable to common shareholders	$ 67,996	$ 63,617	$ 166,424	$ 163,768
Income from continuing operations per share:
Basic	$ 0.38	$ 0.36	$ 0.93	$ 0.89
Diluted	$ 0.38	$ 0.35	$ 0.93	$ 0.89
Income from discontinued operations per share:
Basic	$ 0.00	$ 0.00	$ 0.01	$ 0.04
Diluted	$ 0.00	$ 0.00	$ 0.01	$ 0.04
Net income per common share:
Basic	$ 0.38	$ 0.36	$ 0.94	$ 0.93
Diluted	$ 0.38	$ 0.36	$ 0.94	$ 0.93
Average common shares outstanding during the period:
Basic	176,900	176,483	176,933	175,964
Diluted	177,908	177,575	177,872	176,732
Cash dividends declared per common share	$ 0.165		$ 0.469	$ 0.432