SELECTED CONSOLIDATED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of quarterly unaudited consolidated statements of operations information [Abstract]
Operating revenues	$ 21,008	$ 22,549	$ 20,652	$ 19,386	$ 19,343	$ 21,924	$ 20,238	$ 18,906
Operating income	4,591	5,730	5,320	4,284	4,501	5,244	5,239	3,971
Net income applicable to common stock	$ 3,099	$ 4,458	$ 3,778	$ 3,590	$ 2,945	$ 3,929	$ 3,926	$ 3,478
Income per common share [Abstract]
Basic (in dollars per share)	$ 0.33	$ 0.48	$ 0.41	$ 0.39	$ 0.32	$ 0.42	$ 0.43	$ 0.38	$ 1.61	$ 1.55	$ 1.52
Diluted (in dollars per share)	$ 0.33	$ 0.48	$ 0.41	$ 0.39	$ 0.32	$ 0.42	$ 0.42	$ 0.37	$ 1.60	$ 1.54	$ 1.51