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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, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Summary of quarterly unaudited consolidated statements of operations information [Abstract]                      
Operating revenues $ 18,750 $ 20,776 $ 19,536 $ 17,962 $ 18,079 $ 19,595 $ 17,891 $ 16,900 $ 77,024 $ 72,465 $ 69,073
Operating income 3,531 5,394 4,985 3,672 3,920 5,331 3,649 3,146 17,582 16,046 14,484
Net income applicable to common stock $ 1,835 $ 3,699 $ 3,233 $ 2,538 $ 2,144 $ 3,317 $ 1,962 $ 2,083 $ 11,305 $ 9,506 $ 8,301
Income per common share [Abstract]                      
Basic (in dollars per share) $ 0.21 $ 0.41 $ 0.36 $ 0.28 $ 0.24 $ 0.37 $ 0.22 $ 0.24 $ 1.26 $ 1.07 $ 0.95
Diluted (in dollars per share) $ 0.21 $ 0.41 $ 0.36 $ 0.28 $ 0.24 $ 0.37 $ 0.22 $ 0.23 $ 1.26 $ 1.07 $ 0.94