Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenues	$ 236,796	$ 251,843	$ 412,509	$ 423,178	$ 906,128	$ 934,440
Operating expenses:
Fuel and purchased power	53,463	65,894	105,651	116,173	234,229	248,920
Operations and maintenance	88,855	82,273	169,015	161,460	327,836	321,070
Depreciation and amortization	23,958	22,495	47,772	44,429	94,186	81,601
Taxes other than income taxes	17,381	17,265	32,888	32,995	70,756	68,396
Utilities operating expense	183,657	187,927	355,326	355,057	727,007	719,987
Operating income	53,139	63,916	57,183	68,121	179,121	214,453
Other income (deductions):
Allowance for equity funds used during construction	718	726	1,638	1,541	3,122	4,095
Investment and interest income, net	11,072	12,056	16,227	21,319	33,761	39,251
Miscellaneous non-operating income	3,072	2,897	6,208	5,792	12,466	11,404
Miscellaneous non-operating deductions	(2,769)	(2,669)	(5,512)	(5,497)	(11,594)	(11,404)
Nonoperating income	12,093	13,010	18,561	23,155	37,755	43,346
Interest charges (credits):
Interest on long-term debt and revolving credit facility	18,194	18,407	36,182	36,774	72,378	73,421
Other interest	5,115	4,728	9,769	9,073	18,866	17,473
Capitalized interest	(1,365)	(1,344)	(2,579)	(2,638)	(4,963)	(5,133)
Allowance for borrowed funds used during construction	(772)	(711)	(1,670)	(1,502)	(3,143)	(3,452)
Interest expense	21,172	21,080	41,702	41,707	83,138	82,309
Income before income taxes	44,060	55,846	34,042	49,569	133,738	175,490
Income tax expense	10,765	19,780	7,713	17,492	41,225	63,121
Net income	$ 33,295	$ 36,066	$ 26,329	$ 32,077	$ 92,513	$ 112,369
Basic earnings per share	$ 0.82	$ 0.890	$ 0.650	$ 0.790	$ 2.280	$ 2.770
Diluted earnings per share	0.82	0.890	0.650	0.790	2.270	2.770
Dividends declared per share of common stock	$ 0.360	$ 0.335	$ 0.695	$ 0.645	$ 1.365	$ 1.265
Weighted average number of shares outstanding	40,517,713	40,409,030	40,504,526	40,398,192	40,467,231	40,381,776
Weighted average number of shares and dilutive potential shares outstanding	40,647,799	40,525,585	40,618,045	40,499,344	40,594,049	40,466,995