Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating revenues	$ 251,843	$ 217,865	$ 423,178	$ 375,674	$ 934,440	$ 842,289
Energy expenses:
Fuel	49,173	43,143	85,779	77,462	182,055	178,320
Purchased and interchanged power	16,721	13,610	30,394	23,256	66,865	53,884
Cost of services, energy services	65,894	56,753	116,173	100,718	248,920	232,204
Operating revenues net of energy expenses	185,949	161,112	307,005	274,956	685,520	610,085
Other operating expenses:
Other operations	59,835	56,817	115,958	115,204	242,768	244,899
Maintenance	20,415	20,426	41,405	37,941	70,210	67,747
Depreciation and amortization	22,495	23,852	44,429	47,145	81,601	92,269
Taxes other than income taxes	17,265	15,320	32,995	30,132	68,396	64,277
Utilities operating expense	120,010	116,415	234,787	230,422	462,975	469,192
Operating income	65,939	44,697	72,218	44,534	222,545	140,893
Other income (deductions):
Allowance for equity funds used during construction	726	2,133	1,541	4,469	4,095	8,565
Investment and interest income, net	6,786	3,591	10,772	6,520	18,335	17,376
Miscellaneous non-operating income	39	145	124	801	615	2,176
Miscellaneous non-operating deductions	(530)	(890)	(1,270)	(1,356)	(3,613)	(3,922)
Nonoperating income	7,021	4,979	11,167	10,434	19,432	24,195
Interest charges (credits):
Interest on long-term debt and revolving credit facility	18,407	18,298	36,774	34,897	73,421	67,770
Other interest	762	272	1,182	834	1,651	1,630
Capitalized interest	(1,344)	(1,253)	(2,638)	(2,495)	(5,133)	(4,913)
Allowance for borrowed funds used during construction	(711)	(1,375)	(1,502)	(3,033)	(3,452)	(5,958)
Interest expense	17,114	15,942	33,816	30,203	66,487	58,529
Income before income taxes	55,846	33,734	49,569	24,765	175,490	106,559
Income tax expense	19,780	11,450	17,492	8,289	63,121	32,695
Net income	$ 36,066	$ 22,284	$ 32,077	$ 16,476	$ 112,369	$ 73,864
Basic earnings per share	$ 0.890	$ 0.550	$ 0.790	$ 0.410	$ 2.770	$ 1.830
Diluted earnings per share	0.890	0.550	0.790	0.410	2.770	1.830
Dividends declared per share of common stock	$ 0.335	$ 0.310	$ 0.645	$ 0.605	$ 1.265	$ 1.195
Weighted average number of shares outstanding	40,409,030	40,345,150	40,398,192	40,335,236	40,381,776	40,314,032
Weighted average number of shares and dilutive potential shares outstanding	40,525,585	40,399,491	40,499,344	40,380,640	40,466,995	40,356,239