Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Operating revenues:
Electric	$ 126,282	$ 140,767	$ 268,924	$ 293,856	$ 567,559	$ 574,481
Gas	6,279	6,989	26,096	31,598	46,341	53,369
Other	1,996	2,026	4,081	4,001	8,076	8,148
Total operating revenues	134,557	149,782	299,101	329,455	621,976	635,998
Operating revenue deductions:
Fuel and purchased power	39,153	54,358	88,131	109,944	193,273	198,034
Cost of natural gas sold and transported	2,041	2,678	13,464	17,723	22,767	28,479
Regulated operating expenses	27,669	27,609	56,219	55,566	111,430	107,115
Other operating expenses	789	782	1,594	1,498	3,083	2,974
Maintenance and repairs	15,583	11,393	25,850	21,650	50,975	43,433
Depreciation and amortization	20,127	18,157	40,147	36,097	77,234	71,668
Provision for income taxes	4,056	6,694	12,939	18,868	33,469	41,837
Other taxes	9,092	8,609	19,996	19,119	37,975	36,773
Total operating revenue deductions	118,510	130,280	258,340	280,465	530,206	530,313
Operating income	16,047	19,502	40,761	48,990	91,770	105,685
Other income and (deductions):
Allowance for equity funds used during construction	1,234	1,522	2,212	2,773	5,859	5,234
Interest income	129	3	140	44	147	92
Benefit for other income taxes	14	45	127	99	206	106
Other - non-operating expense, net	(225)	(300)	(924)	(645)	(1,581)	(1,283)
Total other income	1,152	1,270	1,555	2,271	4,631	4,149
Interest charges:
Long-term debt	10,752	10,105	21,503	20,210	41,929	40,424
Short-term debt	95	13	168	18	263	18
Allowance for borrowed funds used during construction	(723)	(830)	(1,287)	(1,570)	(3,214)	(2,880)
Other	305	290	525	504	1,012	1,015
Total interest charges	10,429	9,578	20,909	19,162	39,990	38,577
Net income	$ 6,770	$ 11,194	$ 21,407	$ 32,099	$ 56,411	$ 71,257
Weighted average number of common shares outstanding - basic (in shares)	43,627	43,236	43,579	43,173	43,492	43,048
Weighted average number of common shares outstanding - diluted (in shares)	43,712	43,269	43,669	43,200	43,579	43,069
Total earnings per weighted average share of common stock - basic and diluted (in dollars per share)			$ 0.49	$ 0.74
Total earnings per weighted average share of common stock - basic (in dollars per share)	$ 0.16	$ 0.26			$ 1.30	$ 1.66
Total earnings per weighted average share of common stock - diluted (in dollars per share)	0.15	0.26			1.29	1.65
Dividends declared per share of common stock (in dollars per share)	$ 0.26	$ 0.255	$ 0.52	$ 0.51	$ 1.035	$ 1.015