Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues	$ 139,320	$ 134,557	$ 290,635	$ 299,101	$ 597,107	$ 621,976
Depreciation and amortization	20,817	20,127	41,221	40,147	81,624	77,234
Federal and state income taxes	5,675	4,042	14,314	12,812	35,314	33,263
Operating income	19,377	16,047	42,620	40,761	98,159	91,770
Interest income	44	129	53	140	58	147
Interest expense	11,630	11,152	23,258	22,196	46,164	43,204
Income from AFUDC (debt and equity)	1,678	1,957	4,435	3,499	8,631	9,073
Net income	9,225	6,770	23,234	21,407	58,424	56,411
Capital Expenditures	30,783	47,577	65,232	101,134	140,623	227,165
Total assets	2,468,635		2,468,635		2,468,635		$ 2,446,645
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
Operating Revenues	131,822	126,282	266,185	268,924	552,347	567,559
Depreciation and amortization	19,319	18,690	38,235	37,279	75,689	71,446
Federal and state income taxes	5,659	3,957	13,227	11,159	33,192	30,318
Operating income	18,438	14,954	38,945	36,168	90,900	82,945
Interest income	40	124	43	126	50	131
Interest expense	10,698	10,210	21,386	20,301	42,391	39,391
Income from AFUDC (debt and equity)	1,676	1,955	4,429	3,496	8,614	9,062
Net income	9,207	6,627	21,447	18,723	54,963	51,480
Capital Expenditures	29,483	45,847	62,650	98,110	133,652	219,362
Total assets	2,355,251		2,355,251		2,355,251		2,331,705
Operating segments | Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues	5,641	6,279	20,754	26,096	36,360	46,341
Depreciation and amortization	1,005	976	2,001	1,948	3,975	3,885
Federal and state income taxes	(322)	(332)	477	794	483	1,277
Operating income	435	428	2,728	3,216	4,665	6,093
Interest income	14	16	25	28	33	34
Interest expense	967	965	1,936	1,930	3,873	3,866
Income from AFUDC (debt and equity)	2	2	6	3	17	11
Net income	(530)	(534)	796	1,288	796	2,218
Capital Expenditures	1,141	987	2,003	1,670	5,523	5,051
Total assets	126,037		126,037		126,037		127,359
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
Operating Revenues	2,202	2,341	4,386	4,771	9,780	9,431
Depreciation and amortization	493	461	985	920	1,960	1,903
Federal and state income taxes	338	417	610	859	1,639	1,668
Operating income	504	665	947	1,377	2,594	2,732
Interest income	25	12	49	21	75	35
Net income	548	677	991	1,396	2,665	2,713
Capital Expenditures	159	743	579	1,354	1,448	2,752
Total assets	38,884		38,884		38,884		38,299
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues	(345)	(345)	(690)	(690)	(1,380)	(1,355)
Interest income	(35)	(23)	(64)	(35)	(100)	(53)
Interest expense	(35)	$ (23)	(64)	$ (35)	(100)	$ (53)
Total assets	$ (51,537)		$ (51,537)		$ (51,537)		$ (50,718)