Consolidated Statements Of Operations (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Operating revenues	$ 267,249	$ 307,633	$ 664,079	$ 726,350	$ 855,742	$ 907,694
Energy expenses:
Fuel	56,332	73,034	145,132	177,111	191,528	216,801
Purchased and interchanged power	16,223	25,845	43,304	60,616	57,837	75,904
Cost of services, energy services	72,555	98,879	188,436	237,727	249,365	292,705
Operating revenues net of energy expenses	194,694	208,754	475,643	488,623	606,377	614,989
Other operating expenses:
Other operations	60,906	56,832	174,128	168,148	235,550	230,803
Maintenance	12,831	12,764	43,605	41,760	63,937	57,361
Depreciation and amortization	19,208	20,315	59,329	60,775	79,885	81,650
Taxes other than income taxes	15,353	16,628	43,631	43,131	56,061	56,582
Utilities operating expense	108,298	106,539	320,693	313,814	435,433	426,396
Operating income	86,396	102,215	154,950	174,809	170,944	188,593
Other income (deductions):
Allowance for equity funds used during construction	2,419	1,379	6,589	6,441	8,309	9,612
Investment and interest income, net	1,833	618	3,711	4,593	4,782	6,619
Miscellaneous non-operating income	1,182	113	1,383	384	1,884	1,351
Miscellaneous non-operating deductions	(591)	(648)	(1,494)	(2,061)	(2,620)	(3,990)
Nonoperating income (expense)	4,843	1,462	10,189	9,357	12,355	13,592
Interest charges (credits):
Interest on long-term debt and revolving credit facility	13,659	13,571	40,827	40,595	54,347	54,043
Other interest	387	243	865	777	1,077	918
Capitalized interest	(1,324)	(1,318)	(3,992)	(3,864)	(5,305)	(5,069)
Allowance for borrowed funds used during construction	(1,431)	(808)	(3,894)	(3,837)	(4,905)	(5,857)
Interest expense	11,291	11,688	33,806	33,671	45,214	44,035
Income before income taxes	79,948	91,989	131,333	150,495	138,085	158,150
Income tax expense	28,159	33,668	45,306	52,409	46,605	52,599
Net income	$ 51,789	$ 58,321	$ 86,027	$ 98,086	$ 91,480	$ 105,551
Basic earnings per share	$ 1.29	$ 1.41	$ 2.15	$ 2.33	$ 2.28	$ 2.50
Diluted earnings per share	$ 1.29	$ 1.40	$ 2.14	$ 2.32	$ 2.27	$ 2.49
Dividends declared per share of common stock	$ 0.25	$ 0.22	$ 0.72	$ 0.44	$ 0.94	$ 0.44
Weighted average number of shares outstanding	40,009,866	41,307,632	39,959,866	41,819,428	39,959,034	41,969,628
Weighted average number of shares and dilutive potential shares outstanding	40,091,625	41,564,973	40,044,154	42,051,307	40,085,516	42,207,012