INCOME TAXES (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
Net operating income (loss)	$ 4,352,000	$ 5,945,000	$ 4,669,000	$ 3,853,000	$ 3,531,000	$ 5,394,000	$ 4,985,000	$ 3,672,000	$ 18,819,000	$ 17,582,000	$ 16,046,000
Income tax receivable	150,000				1,428,000				150,000	1,428,000
Federal alternative minimum tax credit carry-forwards	2,474,000				3,971,000				2,474,000	3,971,000	5,459,000
Accrued penalties and interest	4,800				22,000				4,800	22,000
Valuation allowance	286,000				182,000				286,000	182,000	65,000
State income taxes [Abstract]
Current									1,050,000	499,000	173,000
Deferred									860,000	1,299,000	1,231,000
Total state income tax expense									1,910,000	1,798,000	1,404,000
Federal income taxes [Abstract]
Current									1,799,000	2,168,000	(1,073,000)
Deferred									4,622,000	3,818,000	6,044,000
Total federal income tax expense									6,421,000	5,986,000	4,971,000
Reconciliation of effective tax rate [Abstract]
Income before federal and state income taxes									$ 21,285,000	$ 19,088,000	$ 15,881,000
Income before federal and state income taxes									100.00%	100.00%	100.00%
Amount computed at statutory rate									$ 7,237,000	$ 6,490,000	$ 5,400,000
Amount computed at statutory rate									34.00%	34.00%	34.00%
Reconciling Items [Abstract]
State income tax-net of federal tax benefit									$ 1,327,000	$ 1,214,000	$ 918,000
State income tax-net of federal tax benefit									6.20%	6.40%	5.80%
Other									$ (233,000)	$ 80,000	$ 57,000
Other									(1.10%)	0.40%	0.40%
Total income tax expense									$ 8,331,000	$ 7,784,000	$ 6,375,000
Total effective income tax rate									39.10%	40.80%	40.20%
Deferred tax assets related to [Abstract]
Federal alternative minimum tax credit carry-forwards	2,474,000				3,971,000				$ 2,474,000	$ 3,971,000	$ 5,459,000
Federal and state operating loss carry-forwards	752,000				675,000				752,000	675,000	1,045,000
Bad debt allowance	104,000				110,000				104,000	110,000	99,000
Valuation allowance	(286,000)				(182,000)				(286,000)	(182,000)	(65,000)
Stock options	452,000				415,000				452,000	415,000	397,000
Other	320,000				291,000				320,000	291,000	269,000
Total deferred tax assets	3,816,000				5,280,000				3,816,000	5,280,000	7,204,000
Deferred tax liabilities related to [Abstract]
Property plant and equipment basis differences	(70,711,000)				(66,508,000)				(70,711,000)	(66,508,000)	(63,427,000)
Uncertain tax position	(80,000)				(247,000)				(80,000)	(247,000)	(179,000)
Expenses of rate proceedings	(19,000)				(213,000)				(19,000)	(213,000)	(214,000)
Property taxes	(663,000)				(527,000)				(663,000)	(527,000)	(505,000)
Other	(796,000)				(756,000)				(796,000)	(756,000)	(734,000)
Total deferred tax liabilities	(72,269,000)				(68,251,000)				(72,269,000)	(68,251,000)	(65,059,000)
Net deferred tax liability	(68,453,000)				(62,971,000)				(68,453,000)	(62,971,000)	(57,855,000)
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning Of Period				182,000				65,000	182,000	65,000	61,000
Additions Charged to Costs and Expenses									104,000	117,000	4,000
Deductions									0	0	0
Balance at End of Period	286,000				182,000				286,000	182,000	65,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense [Abstract]
Balance at beginning of year				$ 247,000				$ 179,000	247,000	179,000
Additions based on tax positions related to the current year									23,000	51,000
Additions based on tax positions related to prior years									11,000	17,000
Reductions for tax positions of prior years									(201,000)	0
Settlements									0	0
Lapses in Statutes of Limitations									0	0
Balance at end of year	80,000				$ 247,000				$ 80,000	$ 247,000	$ 179,000
Minimum [Member]
Income Tax Examination [Line Items]
Operating Loss Carryforwards Expiration Period									2019
Maximum [Member]
Income Tax Examination [Line Items]
Operating Loss Carryforwards Expiration Period									2037
Federal Authorities [Member]
Income Tax Examination [Line Items]
Federal alternative minimum tax credit carry-forwards	2,500,000								$ 2,500,000
Tax deduction for qualifying capital expenditures									$ 2,700,000
Income tax examination description									for the tax year 2015
Deferred tax assets related to [Abstract]
Federal alternative minimum tax credit carry-forwards	2,500,000								$ 2,500,000
State Authorities [Member]
Income Tax Examination [Line Items]
Net operating loss carry-forwards	$ 13,400,000								$ 13,400,000
Income tax examination description									for the tax years 2013 through 2015
State Authorities [Member] | Minimum [Member]
Income Tax Examination [Line Items]
Open tax year									2013
State Authorities [Member] | Maximum [Member]
Income Tax Examination [Line Items]
Open tax year									2015