Condensed Consolidated Statements Of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Condensed Consolidated Statements Of Comprehensive Income [Abstract]
UTILITY OPERATING REVENUES	$ 321,274	$ 324,478	$ 947,860	$ 918,241
UTILITY OPERATING EXPENSES:
Fuel	94,555	83,795	294,354	242,366
Other operating expenses	58,104	55,224	171,653	162,266
Power purchased	21,344	34,561	62,628	99,080
Maintenance	20,547	20,728	74,654	71,857
Depreciation and amortization	45,953	44,936	136,458	132,211
Taxes other than income taxes	11,228	11,472	34,508	33,643
Income taxes - net	21,824	23,535	51,141	54,057
Total utility operating expenses	273,555	274,251	825,396	795,480
UTILITY OPERATING INCOME	47,719	50,227	122,464	122,761
OTHER INCOME AND (DEDUCTIONS):
Allowance for equity funds used during construction	1,475	259	3,086	650
Miscellaneous income and (deductions) - net	(530)	(733)	(1,715)	(1,339)
Income tax benefit applicable to nonoperating income	5,570	5,284	16,674	15,726
Total other income and (deductions) - net	6,515	4,810	18,045	15,037
INTEREST AND OTHER CHARGES:
Interest on long-term debt	26,047	25,923	78,560	77,635
Other interest	467	499	1,329	1,419
Allowance for borrowed funds used during construction	(868)	(150)	(1,789)	(774)
Amortization of redemption premiums and expense on debt	1,252	1,224	3,824	3,657
Total interest and other charges - net	26,898	27,496	81,924	81,937
NET INCOME	27,336	27,541	58,585	55,861
LESS: PREFERRED DIVIDENDS OF SUBSIDIARY	803	803	2,410	2,410
NET INCOME APPLICABLE TO COMMON STOCK	26,533	26,738	56,175	53,451
COMPREHENSIVE INCOME ATTRIBUTABLE TO COMMON STOCK	$ 26,533	$ 26,738	$ 56,175	$ 53,451