Unaudited Condensed Consolidated Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
UTILITY OPERATING REVENUES	$ 361,339	$ 326,562	$ 1,012,373	$ 951,240
Operation:
Fuel	80,451	85,631	200,115	248,147
Other operating expenses	59,548	58,770	178,991	168,578
Power purchased	39,464	32,606	132,333	106,591
Maintenance	25,918	25,973	92,919	98,010
Depreciation and amortization	54,876	46,662	166,027	136,038
Taxes other than income taxes	11,779	10,061	34,317	33,553
Income taxes - net	29,457	21,404	63,290	48,386
Total utility operating expenses	301,493	281,107	867,992	839,303
UTILITY OPERATING INCOME	59,846	45,455	144,381	111,937
OTHER INCOME AND (DEDUCTIONS):
Allowance for equity funds used during construction	7,077	3,888	20,454	10,185
Gains (Losses) on Extinguishment of Debt	0	(2,728)	0	(22,051)
Miscellaneous income and (deductions) - net	(299)	(556)	(1,433)	(1,924)
Income tax benefit applicable to nonoperating income	4,485	5,521	10,748	24,102
Total other income and (deductions) - net	11,263	6,125	29,769	10,312
INTEREST AND OTHER CHARGES:
Interest on long-term debt	29,018	24,720	82,571	80,061
Other interest	367	539	1,730	1,487
Allowance for borrowed funds used during construction	(6,549)	(3,411)	(18,007)	(8,644)
Amortization of redemption premiums and expense on debt	1,017	1,086	3,123	3,755
Total interest and other charges - net	23,853	22,934	69,417	76,659
NET INCOME (LOSS)	47,256	28,646	104,733	45,590
LESS: PREFERRED DIVIDENDS OF SUBSIDIARY	803	803	2,410	2,410
NET INCOME (LOSS) APPLICABLE TO COMMON STOCK	$ 46,453	$ 27,843	$ 102,323	$ 43,180