Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Operating revenues	$ 323,225	$ 289,713	$ 698,899	$ 672,967	$ 875,801	$ 869,530
Energy expenses:
Fuel	54,355	60,798	131,817	148,340	171,877	207,564
Purchased and interchanged power	24,459	19,520	47,715	42,437	58,823	53,127
Cost of services, energy services	78,814	80,318	179,532	190,777	230,700	260,691
Operating revenues net of energy expenses	244,411	209,395	519,367	482,190	645,101	608,839
Other operating expenses:
Other operations	64,373	65,360	179,577	178,615	243,912	241,510
Maintenance	14,064	14,355	52,005	49,772	67,456	69,782
Depreciation and amortization	15,952	22,380	63,097	67,080	85,841	88,086
Taxes other than income taxes	20,165	19,253	50,297	48,844	65,189	62,711
Utilities operating expense	114,554	121,348	344,976	344,311	462,398	462,089
Operating income	129,857	88,047	174,391	137,879	182,703	146,750
Other income (deductions):
Allowance for equity funds used during construction	1,398	1,874	5,867	8,417	8,089	12,864
Investment and interest income, net	3,773	5,912	10,293	12,564	15,237	17,660
Miscellaneous non-operating income	272	850	1,073	1,537	1,598	3,453
Miscellaneous non-operating deductions	(1,312)	(1,015)	(2,668)	(2,777)	(4,219)	(4,922)
Nonoperating income (expense)	4,131	7,621	14,565	19,741	20,705	29,055
Interest charges (credits):
Interest on long-term debt and revolving credit facility	18,324	16,465	53,221	49,443	69,629	64,668
Other interest	268	424	1,102	941	1,474	1,292
Capitalized interest	(1,243)	(1,208)	(3,738)	(3,758)	(4,948)	(5,053)
Allowance for borrowed funds used during construction	(1,131)	(1,353)	(4,164)	(5,365)	(5,736)	(7,892)
Interest expense	16,218	14,328	46,421	41,261	60,419	53,015
Income before income taxes	117,770	81,340	142,535	116,359	142,989	122,790
Income tax expense	43,134	24,600	51,423	35,089	51,229	37,279
Net income	$ 74,636	$ 56,740	$ 91,112	$ 81,270	$ 91,760	$ 85,511
Basic earnings per share	$ 1.840	$ 1.400	$ 2.250	$ 2.010	$ 2.270	$ 2.120
Diluted earnings per share	1.840	1.400	2.250	2.010	2.270	2.120
Dividends declared per share of common stock	$ 0.310	$ 0.295	$ 0.915	$ 0.870	$ 1.210	$ 1.150
Weighted average number of shares outstanding	40,363,819	40,289,010	40,344,834	40,267,533	40,332,835	40,255,439
Weighted average number of shares and dilutive potential shares outstanding	40,425,942	40,329,529	40,395,811	40,299,801	40,380,443	40,279,640