Stock-Based Compensation (Summary Of Compensation Costs) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation within operations and maintenance expense	$ 30,000	$ 0
Income tax benefit	92,000	216,000
Performance Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation within operations and maintenance expense	870,000	525,000
Income tax benefit	353,000	209,000
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation within operations and maintenance expense	281,000	168,000
Income tax benefit	116,000	69,000
Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation within operations and maintenance expense	131,000	112,000
Income tax benefit	$ 54,000	$ 47,000