Statements Of Operations - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Operating revenues	$ 157,809	$ 163,746	$ 843,932	$ 895,755
Energy expenses:
Fuel	34,319	37,729	184,990	237,148
Purchased and interchanged power	9,646	11,175	52,016	58,064
Cost of services, energy services	43,965	48,904	237,006	295,212
Operating revenues net of energy expenses	113,844	114,842	606,926	600,543
Other operating expenses:
Other operations	58,387	55,599	245,738	238,293
Maintenance	17,515	15,560	67,178	66,907
Depreciation and amortization	23,293	21,565	91,552	84,339
Taxes other than income taxes	14,812	14,158	64,390	61,546
Utilities operating expense	114,007	106,882	468,858	451,085
Operating income (loss)	(163)	7,960	138,068	149,458
Other income (deductions):
Allowance for equity funds used during construction	2,336	4,275	8,700	16,031
Investment and interest income, net	2,929	5,254	15,183	14,646
Miscellaneous non-operating income	656	180	2,538	2,738
Miscellaneous non-operating deductions	(466)	(491)	(4,303)	(4,271)
Nonoperating income (expense)	5,455	9,218	22,118	29,144
Interest charges (credits):
Interest on long-term debt and revolving credit facility	16,599	16,483	65,967	60,932
Other interest	562	163	1,712	1,240
Capitalized interest	(1,242)	(1,289)	(4,921)	(5,135)
Allowance for borrowed funds used during construction	(1,658)	(2,621)	(5,974)	(9,305)
Interest expense	14,261	12,736	56,784	47,732
Income (loss) before income taxes	(8,969)	4,442	103,402	130,870
Income tax expense (benefit)	(3,161)	984	30,750	40,599
Net income (loss)	$ (5,808)	$ 3,458	$ 72,652	$ 90,271
Basic earnings (loss) per share	$ (0.140)	$ 0.090	$ 1.800	$ 2.240
Diluted earnings (loss) per share	(0.140)	0.090	1.800	2.240
Dividends declared per share of common stock	$ 0.295	$ 0.28	$ 1.18	$ 1.12
Weighted average number of shares outstanding	40,325,324	40,243,199	40,295,316	40,214,198
Weighted average number of shares and dilutive potential shares outstanding	40,325,324	40,266,674	40,332,140	40,240,793