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SELECTED CONSOLIDATED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of quarterly unaudited consolidated statements of operations information [Abstract]
Operating revenues	$ 20,187	$ 22,356	$ 20,502	$ 19,190	$ 19,417	$ 21,828	$ 19,395	$ 18,449	$ 82,235	$ 79,089	$ 77,024
Operating income	5,165	5,454	4,990	3,991	4,352	5,945	4,669	3,853	19,600	18,819	17,582
Net income applicable to common stock	$ 3,704	$ 3,942	$ 3,251	$ 3,086	$ 2,723	$ 4,360	$ 3,041	$ 2,830	$ 13,983	$ 12,954	$ 11,305
Income per common share [Abstract]
Basic (in dollars per share)	$ 0.40	$ 0.43	$ 0.35	$ 0.34	$ 0.30	$ 0.48	$ 0.33	$ 0.28	$ 1.52	$ 1.42	$ 1.26
Diluted (in dollars per share)	$ 0.40	$ 0.42	$ 0.35	$ 0.34	$ 0.30	$ 0.48	$ 0.33	$ 0.28	$ 1.51	$ 1.41	$ 1.26

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