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SELECTED CONSOLIDATED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Summary of quarterly unaudited consolidated statements of operations information [Abstract]                      
Operating revenues $ 16,952 $ 19,001 $ 17,912 $ 16,698 $ 16,073 $ 17,730 $ 16,510 $ 14,757 $ 70,563 $ 65,070 $ 64,885
Operating income 3,479 4,495 4,294 3,590 3,562 4,367 3,588 2,219 15,855 13,736 14,268
Net income applicable to common stock $ 1,748 $ 2,846 $ 2,748 $ 2,505 $ 1,746 $ 2,238 $ 1,753 $ 1,009 $ 9,846 $ 6,746 $ 7,620
Income per common share [Abstract]                      
Basic (in dollars per share) $ 0.20 $ 0.33 $ 0.32 $ 0.29 $ 0.20 $ 0.26 $ 0.23 $ 0.13 $ 1.14 $ 0.83 $ 1.01
Diluted (in dollars per share) $ 0.20 $ 0.33 $ 0.32 $ 0.29 $ 0.20 $ 0.26 $ 0.23 $ 0.13 $ 1.13 $ 0.83 $ 1.00