Business Segment Data (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments									2
Revenues	$ 31,808	$ 37,754	$ 32,725	$ 30,579	$ 30,925	$ 34,654	$ 31,666	$ 28,780	$ 132,906	$ 126,025	$ 117,139
Operating Income	7,846	14,156	10,328	8,302	8,460	11,460	9,235	6,685	40,632	35,840	34,392
Depreciation									12,796	12,051	11,444
Other Income (Expense), Net									(862)	293	(403)
Interest Expense									5,293	5,554	5,607
Income Taxes									11,735	10,551	9,937
Net Income	3,220	$ 8,813	$ 5,919	$ 4,790	4,560	$ 6,743	$ 5,089	$ 3,636	22,742	20,028	18,445
Capital Expenditures									47,375	25,773	22,596
Assets	620,161				581,383				620,161	581,383
Regulated [Member]
Segment Reporting Information [Line Items]
Revenues									118,017	111,247	103,556
Operating Income									38,201	33,603	32,000
Depreciation									12,606	11,874	11,262
Other Income (Expense), Net									779	619	157
Interest Expense									5,293	5,554	5,607
Income Taxes									11,091	9,522	8,907
Net Income									22,353	18,889	17,249
Capital Expenditures									47,189	25,706	22,498
Assets	619,915				581,321				619,915	581,321
Non - Regulated [Member]
Segment Reporting Information [Line Items]
Revenues									15,388	15,238	14,143
Operating Income									2,431	2,237	2,392
Depreciation									190	177	182
Other Income (Expense), Net									(1,308)	20	(75)
Interest Expense									89	89	91
Income Taxes									644	1,029	1,030
Net Income									389	1,139	1,196
Capital Expenditures									186	67	98
Assets	6,245				6,436				6,245	6,436
Inter-segment Elimination [Member]
Segment Reporting Information [Line Items]
Revenues									(499)	(460)	(560)
Other Income (Expense), Net									(333)	(346)	(485)
Interest Expense									(89)	(89)	$ (91)
Assets	$ (5,999)				$ (6,374)				$ (5,999)	$ (6,374)