Consolidated Statements Of Net Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Consolidated Statements Of Net Income [Abstract]
Operating revenues	$ 187,787	$ 192,607
Operating expenses:
Operations and maintenance	69,128	73,541
Depreciation	33,837	32,145
Amortization	189	450
Taxes other than income taxes	14,737	14,140
Total operating expenses	117,891	120,276
Operating income	69,896	72,331
Other expense (income):
Interest expense, net	21,326	19,853
Allowance for funds used during construction	(3,193)	(2,308)
Gain on sale of other assets	(269)	(207)
Equity loss in joint venture	30	249
Income before income taxes	52,002	54,744
Provision for income taxes	2,930	3,007
Net income	$ 49,072	$ 51,737
Net income per common share:
Basic	$ 0.28	$ 0.29
Diluted	$ 0.28	$ 0.29
Average common shares outstanding during the period:
Basic	177,479	177,104
Diluted	177,969	177,819
Cash dividends declared per common share	$ 0.1913	$ 0.178