Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Operating revenues	$ 175,713	$ 171,335	$ 921,175	$ 900,462
Energy expenses:
Fuel	41,054	36,606	189,517	176,025
Purchased and interchanged power	11,134	13,673	57,143	63,754
Cost of services, energy services	52,188	50,279	246,660	239,779
Operating revenues net of energy expenses	123,525	121,056	674,515	660,683
Other operating expenses:
Operations and maintenance	80,160	79,187	321,254	317,518
Depreciation and amortization	23,814	21,934	92,723	82,958
Taxes other than income taxes	15,507	15,730	70,640	66,451
Utilities operating expense	119,481	116,851	484,617	466,927
Operating income	4,044	4,205	189,898	193,756
Other income (deductions):
Allowance for equity funds used during construction	920	815	3,130	5,502
Investment and interest income, net	5,155	9,263	34,745	35,959
Miscellaneous non-operating income	3,136	2,895	12,292	10,983
Miscellaneous non-operating deductions	(2,743)	(2,828)	(11,494)	(11,513)
Nonoperating income	6,468	10,145	38,673	40,931
Interest charges (credits):
Interest on long-term debt and revolving credit facility	17,988	18,367	72,591	73,312
Other interest	4,654	4,345	18,479	17,155
Capitalized interest	(1,214)	(1,294)	(4,942)	(5,042)
Allowance for borrowed funds used during construction	(898)	(791)	(3,082)	(4,116)
Interest expense	20,530	20,627	83,046	81,309
Income (loss) before income taxes	(10,018)	(6,277)	145,525	153,378
Income tax (benefit) expense	(3,052)	(2,288)	50,240	54,791
Net income (loss)	$ (6,966)	$ (3,989)	$ 95,285	$ 98,587
Basic earnings (loss) per share	$ (0.170)	$ (0.10)	$ 2.35	$ 2.43
Diluted earnings (loss) per share	(0.170)	(0.10)	2.34	2.43
Dividends declared per share of common stock	$ 0.335	$ 0.310	$ 1.340	$ 1.240
Weighted average number of shares outstanding	40,491,194	40,387,235	40,440,189	40,366,024
Weighted average number of shares and dilutive potential shares outstanding	40,491,194	40,387,235	40,563,625	40,435,689