Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	$ 169,714	$ 171,512	$ 468,815	$ 500,967	$ 620,179	$ 650,025
Depreciation and amortization	20,090	18,550	60,237	54,648	78,774	72,483
Federal and state income taxes	15,264	13,714	28,076	32,484	34,813	41,296
Operating income	35,783	31,709	76,544	80,698	95,845	104,497
Interest income	2	4	142	48	145	92
Interest expense	11,335	10,384	33,531	31,115	44,155	41,487
Income from AFUDC (debt and equity)	2,000	2,771	5,499	7,115	8,301	9,151
Net income	25,285	23,892	46,692	55,991	57,804	71,154
Capital Expenditures	42,346	62,209	143,480	159,117	207,276	201,750
Total assets	2,462,843		2,462,843		2,462,843		$ 2,390,256
Goodwill	39,492		39,492		39,492		39,492
Operating segments | Electric
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	162,412	164,500	431,335	458,355	565,471	588,611
Depreciation and amortization	18,651	17,127	55,930	50,493	72,971	66,937
Federal and state income taxes	15,137	13,560	26,296	30,138	31,894	37,735
Operating income	34,622	30,380	70,791	74,091	87,188	94,978
Interest income	1	2	128	34	131	72
Interest expense	10,384	9,424	30,686	28,245	40,352	37,655
Income from AFUDC (debt and equity)	1,997	2,769	5,493	7,036	8,290	9,062
Net income	25,086	23,561	43,809	52,271	53,005	65,501
Capital Expenditures	39,867	60,151	137,977	151,764	199,189	191,690
Total assets	2,348,920		2,348,920		2,348,920		2,271,539
Operating segments | Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	5,084	4,989	31,181	36,587	46,437	53,406
Depreciation and amortization	984	966	2,932	2,790	3,902	3,720
Federal and state income taxes	(340)	(293)	454	1,065	1,230	1,842
Operating income	413	647	3,629	4,545	5,859	6,735
Interest income	6	4	34	23	36	29
Interest expense	968	968	2,898	2,893	3,865	3,857
Income from AFUDC (debt and equity)	3	2	6	79	11	89
Net income	(560)	(322)	728	1,712	1,981	2,932
Capital Expenditures	2,009	1,628	3,679	6,084	5,431	7,679
Total assets	124,499		124,499		124,499		130,856
Goodwill	39,492		39,492		39,492		39,492
Operating segments | Other
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	2,563	2,343	7,334	6,985	9,650	9,288
Depreciation and amortization	455	457	1,375	1,365	1,901	1,826
Federal and state income taxes	467	447	1,326	1,281	1,689	1,719
Operating income	748	682	2,124	2,062	2,798	2,784
Interest income	12	6	33	14	40	16
Net income	759	653	2,155	2,008	2,818	2,721
Capital Expenditures	470	430	1,824	1,269	2,656	2,381
Total assets	36,130		36,130		36,130		34,655
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Revenues	(345)	(320)	(1,035)	(960)	(1,379)	(1,280)
Interest income	(17)	(8)	(53)	(23)	(62)	(25)
Interest expense	(17)	$ (8)	(53)	$ (23)	(62)	$ (25)
Total assets	$ (46,706)		$ (46,706)		$ (46,706)		$ (46,794)