Unaudited Condensed Consolidated Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
UTILITY OPERATING REVENUES	$ 355,314	$ 361,339	$ 1,014,048	$ 1,012,373
Operation:
Fuel	77,061	80,451	210,521	200,115
Other operating expenses	62,776	59,548	188,288	178,991
Power purchased	44,470	39,464	141,058	132,333
Maintenance	29,706	25,918	96,032	92,919
Depreciation and amortization	52,711	54,876	156,099	166,027
Taxes other than income taxes	11,804	11,779	33,640	34,317
Income taxes - net	23,049	29,457	53,189	63,290
Total utility operating expenses	301,577	301,493	878,827	867,992
UTILITY OPERATING INCOME	53,737	59,846	135,221	144,381
OTHER INCOME AND (DEDUCTIONS):
Allowance for equity funds used during construction	6,628	7,077	19,576	20,454
Gain (Loss) on Early Extinguishment of Debt	(8,875)	0	(8,875)	0
Miscellaneous income and (deductions) - net	(185)	(299)	(728)	(1,433)
Income tax benefit applicable to nonoperating income	8,308	4,485	15,492	10,748
Total other income and (deductions) - net	5,876	11,263	25,465	29,769
INTEREST AND OTHER CHARGES:
Interest on long-term debt	30,575	29,018	89,077	82,571
Other interest	466	367	1,263	1,730
Allowance for borrowed funds used during construction	(5,702)	(6,549)	(16,763)	(18,007)
Amortization of redemption premiums and expense on debt	1,081	1,017	3,241	3,123
Total interest and other charges - net	26,420	23,853	76,818	69,417
NET INCOME	33,193	47,256	83,868	104,733
LESS: PREFERRED DIVIDENDS OF SUBSIDIARY	803	803	2,410	2,410
NET INCOME APPLICABLE TO COMMON STOCK	$ 32,390	$ 46,453	$ 81,458	$ 102,323