Consolidated Statements Of Operations (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Operating revenues	$ 282,661	$ 267,249	$ 700,065	$ 664,079	$ 888,867	$ 855,742
Energy expenses:
Fuel	69,268	56,332	175,097	145,132	221,041	191,528
Purchased and interchanged power	17,395	16,223	46,185	43,304	63,132	57,837
Cost of services, energy services	86,663	72,555	221,282	188,436	284,173	249,365
Operating revenues net of energy expenses	195,998	194,694	478,783	475,643	604,694	606,377
Other operating expenses:
Other operations	58,810	60,906	173,810	174,128	236,240	235,550
Maintenance	12,723	12,831	41,260	43,605	57,994	63,937
Depreciation and amortization	20,416	19,208	59,346	59,329	78,573	79,885
Taxes other than income taxes	18,153	15,353	44,782	43,631	58,594	56,061
Utilities operating expense	110,102	108,298	319,198	320,693	431,401	435,433
Operating income	85,896	86,396	159,585	154,950	173,293	170,944
Other income (deductions):
Allowance for equity funds used during construction	2,287	2,419	7,465	6,589	10,303	8,309
Investment and interest income, net	2,086	1,833	5,151	3,711	6,715	4,782
Miscellaneous non-operating income	12	1,182	14	1,383	46	1,884
Miscellaneous non-operating deductions	(537)	(591)	(2,641)	(1,494)	(3,160)	(2,620)
Nonoperating income (expense)	3,848	4,843	9,989	10,189	13,904	12,355
Interest charges (credits):
Interest on long-term debt and revolving credit facility	14,623	13,659	43,829	40,827	57,634	54,347
Other interest	153	387	456	865	781	1,077
Capitalized interest	(1,302)	(1,324)	(3,911)	(3,992)	(5,231)	(5,305)
Allowance for borrowed funds used during construction	(1,379)	(1,431)	(4,520)	(3,894)	(6,199)	(4,905)
Interest expense	12,095	11,291	35,854	33,806	46,985	45,214
Income before income taxes	77,649	79,948	133,720	131,333	140,212	138,085
Income tax expense	27,084	28,159	46,328	45,306	48,001	46,605
Net income	$ 50,565	$ 51,789	$ 87,392	$ 86,027	$ 92,211	$ 91,480
Basic earnings per share	$ 1.260	$ 1.290	$ 2.170	$ 2.150	$ 2.290	$ 2.280
Diluted earnings per share	$ 1.260	$ 1.290	$ 2.170	$ 2.140	$ 2.290	$ 2.270
Dividends declared per share of common stock	$ 0.265	$ 0.250	$ 0.780	$ 0.720	$ 1.030	$ 0.940
Weighted average number of shares outstanding	40,132,250	40,009,866	40,107,555	39,959,866	40,084,525	39,959,034
Weighted average number of shares and dilutive potential shares outstanding	40,132,250	40,091,625	40,123,626	40,044,154	40,114,921	40,085,516