Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Operating revenues	$ 289,713	$ 283,645	$ 672,967	$ 720,962	$ 869,530	$ 911,259
Energy expenses:
Fuel	60,798	70,523	148,340	191,781	207,564	243,452
Purchased and interchanged power	19,520	18,071	42,437	54,114	53,127	70,292
Cost of services, energy services	80,318	88,594	190,777	245,895	260,691	313,744
Operating revenues net of energy expenses	209,395	195,051	482,190	475,067	608,839	597,515
Other operating expenses:
Other operations	65,360	59,514	178,615	175,937	241,510	239,282
Maintenance	14,355	15,392	49,772	45,619	69,782	65,427
Depreciation and amortization	22,380	20,685	67,080	62,336	88,086	82,616
Taxes other than income taxes	19,253	17,964	48,844	48,883	62,711	61,848
Utilities operating expense	121,348	113,555	344,311	332,775	462,089	449,173
Operating income	88,047	81,496	137,879	142,292	146,750	148,342
Other income (deductions):
Allowance for equity funds used during construction	1,874	3,848	8,417	10,215	12,864	12,758
Investment and interest income, net	5,912	2,373	12,564	8,537	17,660	10,419
Miscellaneous non-operating income	850	52	1,537	2,159	3,453	3,054
Miscellaneous non-operating deductions	(1,015)	(1,036)	(2,777)	(2,054)	(4,922)	(3,048)
Nonoperating income (expense)	7,621	5,237	19,741	18,857	29,055	23,183
Interest charges (credits):
Interest on long-term debt and revolving credit facility	16,465	14,617	49,443	43,803	64,668	58,609
Other interest	424	438	941	899	1,292	874
Capitalized interest	(1,208)	(1,270)	(3,758)	(3,797)	(5,053)	(5,185)
Allowance for borrowed funds used during construction	(1,353)	(2,190)	(5,365)	(5,841)	(7,892)	(7,376)
Interest expense	14,328	11,595	41,261	35,064	53,015	46,922
Income before income taxes	81,340	75,138	116,359	126,085	122,790	124,603
Income tax expense	24,600	22,662	35,089	38,898	37,279	36,225
Net income	$ 56,740	$ 52,476	$ 81,270	$ 87,187	$ 85,511	$ 88,378
Basic earnings per share	$ 1.400	$ 1.300	$ 2.010	$ 2.160	$ 2.120	$ 2.190
Diluted earnings per share	1.400	1.300	2.010	2.160	2.120	2.190
Dividends declared per share of common stock	$ 0.295	$ 0.280	$ 0.870	$ 0.825	$ 1.150	$ 1.090
Weighted average number of shares outstanding	40,289,010	40,213,741	40,267,533	40,181,367	40,255,439	40,169,801
Weighted average number of shares and dilutive potential shares outstanding	40,329,529	40,264,810	40,299,801	40,209,001	40,279,640	40,190,527