Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Operating revenues	$ 219,508	$ 251,801	$ 383,254	$ 437,317	$ 863,462	$ 910,275
Energy expenses:
Fuel	49,813	69,672	87,542	121,258	217,289	242,197
Purchased and interchanged power	11,742	18,128	22,917	36,043	51,678	69,616
Cost of services, energy services	61,555	87,800	110,459	157,301	268,967	311,813
Operating revenues net of energy expenses	157,953	164,001	272,795	280,016	594,495	598,462
Other operating expenses:
Other operations	57,656	60,285	113,255	116,423	235,664	238,578
Maintenance	19,857	15,945	35,417	30,227	70,819	62,758
Depreciation and amortization	23,135	21,083	44,700	41,651	86,391	82,347
Taxes other than income taxes	15,433	15,557	29,591	30,919	61,422	62,037
Utilities operating expense	116,081	112,870	222,963	219,220	454,296	445,720
Operating income	41,872	51,131	49,832	60,796	140,199	152,742
Other income (deductions):
Allowance for equity funds used during construction	2,268	3,461	6,543	6,367	14,838	11,197
Investment and interest income, net	1,398	1,923	6,652	6,164	14,121	10,132
Miscellaneous non-operating income	507	590	687	2,107	2,655	3,014
Miscellaneous non-operating deductions	(1,271)	(599)	(1,762)	(1,018)	(4,943)	(2,549)
Nonoperating income (expense)	2,902	5,375	12,120	13,620	26,671	21,794
Interest charges (credits):
Interest on long-term debt and revolving credit facility	16,495	14,607	32,978	29,186	62,820	58,615
Other interest	354	288	517	461	1,306	589
Capitalized interest	(1,261)	(1,281)	(2,550)	(2,527)	(5,115)	(5,217)
Allowance for borrowed funds used during construction	(1,391)	(1,967)	(4,012)	(3,651)	(8,729)	(6,565)
Interest expense	14,197	11,647	26,933	23,469	50,282	47,422
Income before income taxes	30,577	44,859	35,019	50,947	116,588	127,114
Income tax expense	9,505	14,763	10,489	16,236	35,341	40,647
Net income	$ 21,072	$ 30,096	$ 24,530	$ 34,711	$ 81,247	$ 86,467
Basic earnings per share	$ 0.520	$ 0.750	$ 0.610	$ 0.860	$ 2.010	$ 2.150
Diluted earnings per share	0.520	0.750	0.610	0.860	2.010	2.150
Dividends declared per share of common stock	$ 0.295	$ 0.280	$ 0.575	$ 0.545	$ 1.135	$ 1.075
Weighted average number of shares outstanding	40,269,885	40,180,569	40,256,615	40,164,913	40,236,466	40,149,261
Weighted average number of shares and dilutive potential shares outstanding	40,302,694	40,212,403	40,284,757	40,180,830	40,263,304	40,157,220