SELECTED CONSOLIDATED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	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
Summary of quarterly unaudited consolidated statements of operations information [Abstract]
Operating revenues	$ 19,417	$ 21,828	$ 19,395	$ 18,449	$ 18,750	$ 20,776	$ 19,536	$ 17,962	$ 79,089	$ 77,024	$ 72,465
Operating income	4,352	5,945	4,669	3,853	3,531	5,394	4,985	3,672	18,819	17,582	16,046
Net income applicable to common stock	$ 2,723	$ 4,360	$ 3,041	$ 2,830	$ 1,835	$ 3,699	$ 3,233	$ 2,538	$ 12,954	$ 11,305	$ 9,506
Income per common share [Abstract]
Basic (in dollars per share)	$ 0.30	$ 0.48	$ 0.33	$ 0.31	$ 0.21	$ 0.41	$ 0.36	$ 0.28	$ 1.42	$ 1.26	$ 1.07
Diluted (in dollars per share)	$ 0.30	$ 0.48	$ 0.33	$ 0.31	$ 0.21	$ 0.41	$ 0.36	$ 0.28	$ 1.41	$ 1.26	$ 1.07