Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating revenues	$ 294,453	$ 300,271	$ 671,891	$ 712,780	$ 862,714	$ 908,929
Operating expenses:
Fuel and purchased power	50,583	71,086	131,015	176,737	183,387	229,996
Operations and maintenance	82,186	83,355	243,907	252,370	326,420	336,506
Depreciation and amortization	25,704	24,169	76,050	71,941	100,491	95,790
Taxes other than income taxes	21,953	21,728	56,245	54,616	72,629	71,271
Utilities operating expense	180,426	200,338	507,217	555,664	682,927	733,563
Operating income	114,027	99,933	164,674	157,116	179,787	175,366
Other income (deductions):
Allowance for equity funds used during construction	481	824	1,996	2,462	2,987	3,278
Investment and interest income, net	11,904	16,815	48,625	33,042	33,960	41,587
Miscellaneous non-operating income	3,150	3,037	9,288	9,245	12,866	12,428
Strategic transaction costs	(3,798)	0	(9,473)	0	(9,473)	0
Miscellaneous non-operating deductions	(2,930)	(3,263)	(7,615)	(8,775)	(10,820)	(11,677)
Nonoperating income	8,807	17,413	42,821	35,974	29,520	45,616
Interest charges (credits):
Interest on long-term debt and revolving credit facility	18,772	19,603	56,024	55,785	75,663	73,766
Other interest	5,314	4,127	16,357	13,896	20,351	18,320
Capitalized interest	(1,435)	(1,488)	(4,424)	(4,067)	(5,840)	(5,258)
Allowance for borrowed funds used during construction	(986)	(881)	(3,020)	(2,551)	(4,081)	(3,353)
Interest expense	21,665	21,361	64,937	63,063	86,093	83,475
Income before income taxes	101,169	95,985	142,558	130,027	123,214	137,507
Income tax expense	23,289	22,714	32,463	30,427	28,404	31,407
Net income	$ 77,880	$ 73,271	$ 110,095	$ 99,600	$ 94,810	$ 106,100
Basic earnings per share	$ 1.910	$ 1.80	$ 2.70	$ 2.450	$ 2.33	$ 2.61
Diluted earnings per share	1.910	1.79	2.70	2.440	2.32	2.60
Dividends declared per share of common stock	$ 0.385	$ 0.360	$ 1.130	$ 1.055	$ 1.49	$ 1.39
Weighted average number of shares outstanding	40,616,532	40,535,489	40,600,726	40,514,961	40,585,513	40,494,482
Weighted average number of shares and dilutive potential shares outstanding	40,679,868	40,697,047	40,679,998	40,644,493	40,669,094	40,630,758