Business Segment Information (Summary Of Company's Reporting Segments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 1,349,588	$ 1,347,430	$ 1,250,399
Depreciation and amortization	208,204	218,444	188,267
Tax benefit	48,951	61,209	31,566
Net income	108,793	131,060	59,524
Utility plant - net of depreciation	3,980,991	3,880,918	3,441,000
Capital expenditures	229,000	608,000	686,000
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,350,000	1,347,000	1,250,000
Depreciation and amortization	208,000	218,000	188,000
Tax benefit	66,000	73,000	56,000
Net income	137,000	156,000	102,000
Utility plant - net of depreciation	3,981,000	3,881,000	3,441,000
Capital expenditures	229,000	608,000	686,000
Other Segments [Member]
Segment Reporting Information [Line Items]
Tax benefit	(17,000)	(11,000)	(25,000)
Net income	$ (28,000)	$ (25,000)	$ (42,000)