SELECTED CONSOLIDATED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary of quarterly unaudited consolidated statements of operations information [Abstract]
Operating revenues	$ 21,751	$ 24,737	$ 21,752	$ 19,901	$ 21,008	$ 22,549	$ 20,652	$ 19,386
Operating income	4,840	6,869	6,124	4,462	4,591	5,730	5,320	4,284
Net income applicable to common stock	$ 3,114	$ 5,063	$ 4,566	$ 4,074	$ 3,099	$ 4,458	$ 3,778	$ 3,590
Income per common share [Abstract]
Basic (in dollars per share)	$ 0.33	$ 0.54	$ 0.49	$ 0.44	$ 0.33	$ 0.48	$ 0.41	$ 0.39	$ 1.80	$ 1.61	$ 1.55
Diluted (in dollars per share)	$ 0.33	$ 0.54	$ 0.49	$ 0.44	$ 0.33	$ 0.48	$ 0.41	$ 0.39	$ 1.79	$ 1.60	$ 1.54