Business Segment Data (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments									2
Revenues	$ 34,626	$ 39,920	$ 35,277	$ 31,769	$ 32,738	$ 37,769	$ 33,393	$ 30,698	$ 141,592	$ 134,598	$ 138,077
Operating Income	8,331	13,177	9,385	6,527	7,559	11,983	8,950	7,028	37,420	35,520	37,142
Depreciation									18,472	16,716	15,037
Other Income (Expense), Net									4,379	2,492	2,992
Interest Expense									7,493	7,264	6,758
Income Taxes									(4,119)	(3,140)	924
Net Income	8,307	$ 12,737	$ 9,713	$ 7,668	8,071	$ 11,119	$ 8,146	$ 6,552	38,425	33,888	32,452
Capital Expenditures									105,619	89,125	72,094
Assets	976,470				909,878				976,470	909,878
Inter-segment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues									(804)	(657)	(602)
Other Income (Expense), Net									(356)	(273)	(411)
Interest Expense									(357)	(273)	(411)
Assets	(30,751)				(9,889)				(30,751)	(9,889)
Regulated [Member]
Segment Reporting Information [Line Items]
Revenues									129,851	123,078	121,901
Operating Income									34,043	31,805	34,127
Depreciation									18,264	16,481	14,846
Other Income (Expense), Net									4,605	3,018	3,284
Interest Expense									7,780	7,456	7,060
Income Taxes									(5,139)	(4,317)	(54)
Net Income									35,951	31,602	30,405
Capital Expenditures									105,091	88,858	71,493
Assets	998,932				910,081				998,932	910,081
Non - Regulated [Member]
Segment Reporting Information [Line Items]
Revenues									12,545	12,177	16,778
Operating Income									3,377	3,715	3,015
Depreciation									208	235	191
Other Income (Expense), Net									130	(253)	119
Interest Expense									70	81	109
Income Taxes									1,020	1,177	978
Net Income									2,474	2,286	2,047
Capital Expenditures									528	267	$ 601
Assets	$ 8,289				$ 9,686				$ 8,289	$ 9,686