Segment Information (Summary Of Company's Reporting Segments) (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Operating revenues	$ 311,536,000	$ 324,478,000	$ 292,659,000	$ 301,104,000	$ 282,266,000	$ 320,550,000	$ 279,943,000	$ 289,165,000	$ 1,229,777,000	$ 1,171,924,000	$ 1,144,903,000
Depreciation and amortization									176,843,000	167,245,000	164,102,000
Income taxes									47,699,000	37,133,000	50,529,000
Net income	16,135,000	27,541,000	12,291,000	16,029,000	8,491,000	26,120,000	12,534,000	13,430,000	71,996,000	60,575,000	79,947,000
Utility plant - net	2,425,610,000				2,441,347,000				2,425,610,000	2,441,347,000
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,230,000,000	1,172,000,000	1,145,000,000
Depreciation and amortization									177,000,000	167,000,000	164,000,000
Income taxes									68,000,000	66,000,000	76,000,000
Net income									104,000,000	105,000,000	120,000,000
Utility plant - net	2,426,000,000				2,441,000,000				2,426,000,000	2,441,000,000	2,362,000,000
Capital expenditures									130,000,000	210,000,000	164,000,000
All Other Segments [Member]
Segment Reporting Information [Line Items]
Income taxes									(20,000,000)	(29,000,000)	(25,000,000)
Net income									$ (32,000,000)	$ (44,000,000)	$ (40,000,000)