Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating revenues	$ 300,271	$ 297,470	$ 712,780	$ 720,648	$ 908,929	$ 908,685
Operating expenses:
Fuel and purchased power	71,086	75,319	176,737	191,492	229,996	245,425
Operations and maintenance	83,355	74,685	252,370	236,145	336,506	315,677
Depreciation and amortization	24,169	22,565	71,941	66,994	95,790	88,214
Taxes other than income taxes	21,728	21,213	54,616	54,208	71,271	69,444
Utilities operating expense	200,338	193,782	555,664	548,839	733,563	718,760
Operating income	99,933	103,688	157,116	171,809	175,366	189,925
Other income (deductions):
Allowance for equity funds used during construction	824	668	2,462	2,209	3,278	3,365
Investment and interest income, net	16,815	8,989	33,042	30,308	41,587	39,281
Miscellaneous non-operating income	3,037	3,075	9,245	8,867	12,428	11,901
Miscellaneous non-operating deductions	(3,263)	(3,180)	(8,775)	(8,677)	(11,677)	(11,542)
Nonoperating income	17,413	9,552	35,974	32,707	45,616	43,005
Interest charges (credits):
Interest on long-term debt and revolving credit facility	19,603	18,215	55,785	54,989	73,766	73,312
Other interest	4,127	4,673	13,896	13,746	18,320	17,757
Capitalized interest	(1,488)	(1,193)	(4,067)	(3,831)	(5,258)	(5,083)
Allowance for borrowed funds used during construction	(881)	(671)	(2,551)	(2,173)	(3,353)	(2,992)
Interest expense	21,361	21,024	63,063	62,731	83,475	82,994
Income before income taxes	95,985	92,216	130,027	141,785	137,507	149,936
Income tax expense	22,714	32,532	30,427	50,024	31,407	52,519
Net income	$ 73,271	$ 59,684	$ 99,600	$ 91,761	$ 106,100	$ 97,417
Basic earnings per share	$ 1.80	$ 1.470	$ 2.450	$ 2.26	$ 2.61	$ 2.40
Diluted earnings per share	1.79	1.470	2.440	2.26	2.60	2.40
Dividends declared per share of common stock	$ 0.360	$ 0.335	$ 1.055	$ 0.980	$ 1.390	$ 1.290
Weighted average number of shares outstanding	40,535,489	40,427,589	40,514,961	40,408,100	40,494,482	40,398,022
Weighted average number of shares and dilutive potential shares outstanding	40,697,047	40,551,315	40,644,493	40,516,777	40,630,758	40,498,642