Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating revenues	$ 203,075	$ 236,796	$ 377,438	$ 412,509	$ 868,532	$ 906,128
Operating expenses:
Fuel and purchased power	32,106	53,463	80,432	105,651	203,890	234,229
Operations and maintenance	81,308	88,855	161,721	169,015	327,589	327,836
Depreciation and amortization	25,220	23,958	50,346	47,772	98,956	94,186
Taxes other than income taxes	18,103	17,381	34,292	32,888	72,404	70,756
Utilities operating expense	156,737	183,657	326,791	355,326	702,839	727,007
Operating income	46,338	53,139	50,647	57,183	165,693	179,121
Other income (deductions):
Allowance for equity funds used during construction	514	718	1,515	1,638	3,330	3,122
Investment and interest income, net	13,014	11,072	36,721	16,227	38,871	33,761
Miscellaneous non-operating income	3,090	3,072	6,138	6,208	12,753	12,466
Strategic transaction costs	(5,675)	0	(5,675)	0	(5,675)	0
Miscellaneous non-operating deductions	(2,328)	(2,769)	(4,685)	(5,512)	(11,153)	(11,594)
Nonoperating income	8,615	12,093	34,014	18,561	38,126	37,755
Interest charges (credits):
Interest on long-term debt and revolving credit facility	18,263	18,194	37,252	36,182	76,494	72,378
Other interest	5,810	5,115	11,043	9,769	19,164	18,866
Capitalized interest	(1,457)	(1,365)	(2,989)	(2,579)	(5,893)	(4,963)
Allowance for borrowed funds used during construction	(1,062)	(772)	(2,034)	(1,670)	(3,976)	(3,143)
Interest expense	21,554	21,172	43,272	41,702	85,789	83,138
Income before income taxes	33,399	44,060	41,389	34,042	118,030	133,738
Income tax expense	7,273	10,765	9,174	7,713	27,829	41,225
Net income	$ 26,126	$ 33,295	$ 32,215	$ 26,329	$ 90,201	$ 92,513
Basic earnings per share	$ 0.640	$ 0.82	$ 0.790	$ 0.650	$ 2.220	$ 2.280
Diluted earnings per share	0.640	0.82	0.790	0.650	2.210	2.270
Dividends declared per share of common stock	$ 0.385	$ 0.360	$ 0.745	$ 0.695	$ 1.465	$ 1.365
Weighted average number of shares outstanding	40,602,343	40,517,713	40,592,693	40,504,526	40,565,007	40,467,231
Weighted average number of shares and dilutive potential shares outstanding	40,696,007	40,647,799	40,679,934	40,618,045	40,673,144	40,594,049