Consolidated Statements Of Net Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Consolidated Statements Of Net Income [Abstract]
Operating revenues	$ 190,326us-gaap_Revenues	$ 182,672us-gaap_Revenues
Operating expenses:
Operations and maintenance	73,189	71,686
Depreciation	30,500us-gaap_Depreciation	30,981us-gaap_Depreciation
Amortization	849us-gaap_AdjustmentForAmortization	1,133us-gaap_AdjustmentForAmortization
Taxes other than income taxes	14,621us-gaap_UtilitiesOperatingExpenseTaxes	12,102us-gaap_UtilitiesOperatingExpenseTaxes
Total operating expenses	119,159us-gaap_CostsAndExpenses	115,902us-gaap_CostsAndExpenses
Operating income	71,167us-gaap_OperatingIncomeLoss	66,770us-gaap_OperatingIncomeLoss
Other expense (income):
Interest expense, net	18,665us-gaap_InterestExpense	19,310us-gaap_InterestExpense
Allowance for funds used during construction	(1,182)	(1,167)
(Gain) loss on sale of other assets	(169)us-gaap_GainLossOnSaleOfOtherAssets	348us-gaap_GainLossOnSaleOfOtherAssets
Equity loss in joint venture	714	686
Income from continuing operations before income taxes	53,139	47,593
Provision for income taxes	4,594us-gaap_IncomeTaxExpenseBenefit	5,192us-gaap_IncomeTaxExpenseBenefit
Income from continuing operations	48,545	42,401
Discontinued operations:
Income from discontinued operations before income taxes		772
Provision for income taxes		314
Income from discontinued operations		458
Net income attributable to common shareholders	$ 48,545us-gaap_NetIncomeLoss	$ 42,859us-gaap_NetIncomeLoss
Income from continuing operations per share:
Basic	$ 0.27	$ 0.24
Diluted	$ 0.27	$ 0.24
Income from discontinued operations per share:
Basic		$ 0.00
Diluted		$ 0.00
Net income per common share:
Basic	$ 0.27us-gaap_EarningsPerShareBasic	$ 0.24us-gaap_EarningsPerShareBasic
Diluted	$ 0.27us-gaap_EarningsPerShareDiluted	$ 0.24us-gaap_EarningsPerShareDiluted
Average common shares outstanding during the period:
Basic	176,888	176,839
Diluted	177,792	177,810
Cash dividends declared per common share	$ 0.165	$ 0.152

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