SEGMENT INFORMATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	$ 164,544	$ 179,673	$ 637,201	$ 622,863
Depreciation and amortization	20,020	17,940	75,264	71,146
Federal and state income taxes	8,771	12,121	35,871	42,131
Operating income	24,713	29,488	95,225	107,293
Interest income	11	41	20	100
Interest expense	11,044	10,325	42,459	41,554
Income from AFUDC (debt and equity)	1,542	1,993	9,467	7,101
Net income	14,637	20,905	60,835	71,721
Capital Expenditures	53,555	50,332	226,347	173,194
Total assets	2,407,579		2,407,579		2,390,256
Goodwill	39,492		39,492		39,492
Operating segments | Electric
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	142,641	153,089	582,043	560,740
Depreciation and amortization	18,589	16,575	69,547	65,552
Federal and state income taxes	7,202	10,247	32,692	38,731
Operating income	21,214	25,526	86,176	97,995
Interest income	2	31	7	73
Interest expense	10,091	9,367	38,635	37,714
Income from AFUDC (debt and equity)	1,541	1,967	9,408	7,045
Net income	12,096	17,884	55,679	66,263
Capital Expenditures	52,263	46,703	218,696	163,930
Total assets	2,291,695		2,291,695		2,271,539
Operating segments | Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	19,818	24,609	47,051	54,157
Depreciation and amortization	972	910	3,822	3,695
Federal and state income taxes	1,126	1,448	1,519	1,726
Operating income	2,788	3,274	6,289	6,573
Interest income	12	12	25	56
Interest expense	965	964	3,863	3,876
Income from AFUDC (debt and equity)	1	26	59	56
Net income	1,822	2,330	2,457	2,737
Capital Expenditures	683	3,172	5,348	6,859
Total assets	131,432		131,432		130,856
Goodwill	39,492		39,492		39,492
Operating segments | Others
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	2,430	2,295	9,437	9,410
Depreciation and amortization	459	455	1,895	1,899
Federal and state income taxes	443	426	1,660	1,674
Operating income	711	688	2,760	2,725
Interest income	9	4	27	7
Net income	719	691	2,699	2,721
Capital Expenditures	609	457	2,303	2,405
Total assets	34,961		34,961		34,655
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	(345)	(320)	(1,330)	(1,444)
Interest income	(12)	(6)	(39)	(36)
Interest expense	(12)	(6)	(39)	(36)
Total assets	$ (50,509)		$ (50,509)		$ (46,794)