Statements Of Operations (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Operating revenues	$ 185,516	$ 177,290	$ 898,588	$ 861,593
Energy expenses:
Fuel	51,586	44,399	233,955	196,041
Purchased and interchanged power	17,915	12,877	67,401	60,569
Cost of services, energy services	69,501	57,276	301,356	256,610
Operating revenues net of energy expenses	116,015	120,014	597,232	604,983
Other operating expenses:
Other operations	56,138	55,967	237,326	238,108
Maintenance	14,282	12,552	62,798	56,923
Depreciation and amortization	20,568	19,368	80,826	77,406
Taxes other than income taxes	15,362	12,782	60,327	56,585
Utilities operating expense	106,350	100,669	441,277	429,022
Operating income	9,665	19,345	155,955	175,961
Other income (deductions):
Allowance for equity funds used during construction	2,906	2,663	10,251	10,134
Investment and interest income, net	4,241	1,231	10,043	4,730
Miscellaneous non-operating income	1,517	1	2,425	1,346
Miscellaneous non-operating deductions	(419)	(471)	(3,583)	(2,002)
Nonoperating income (expense)	8,245	3,424	19,136	14,208
Interest charges (credits):
Interest on long-term debt and revolving credit facility	14,579	14,596	58,618	55,665
Other interest	173	149	455	1,139
Capitalized interest	(1,246)	(1,302)	(5,243)	(5,245)
Allowance for borrowed funds used during construction	(1,684)	(1,623)	(6,116)	(6,043)
Interest expense	11,822	11,820	47,714	45,516
Income before income taxes	6,088	10,949	127,377	144,653
Income tax expense	1,473	3,315	41,813	49,517
Net income	$ 4,615	$ 7,634	$ 85,564	$ 95,136
Basic earnings per share	$ 0.110	$ 0.190	$ 2.130	$ 2.370
Diluted earnings per share	$ 0.110	$ 0.190	$ 2.120	$ 2.370
Dividends declared per share of common stock	$ 0.265	$ 0.250	$ 1.060	$ 1.000
Weighted average number of shares outstanding	40,149,083	40,078,061	40,132,106	40,015,380
Weighted average number of shares and dilutive potential shares outstanding	40,149,083	40,078,061	40,144,159	40,074,820