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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, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Summary of quarterly unaudited consolidated statements of operations information [Abstract] |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 19,343 | $ 21,924 | $ 20,238 | $ 18,906 | $ 20,187 | $ 22,356 | $ 20,502 | $ 19,190 |  |  |
Operating income | 4,501 | 5,244 | 5,239 | 3,971 | 5,165 | 5,454 | 4,990 | 3,991 | $ 18,955 | $ 19,600 | $ 18,819
Net income applicable to common stock | $ 2,945 | $ 3,929 | $ 3,926 | $ 3,478 | $ 3,704 | $ 3,942 | $ 3,251 | $ 3,086 | $ 14,278 | $ 13,983 | $ 12,954
Income per common share [Abstract] |  |  |  |  |  |  |  |  |  |  |
Basic (in dollars per share) | $ 0.32 | $ 0.42 | $ 0.43 | $ 0.38 | $ 0.40 | $ 0.43 | $ 0.35 | $ 0.34 | $ 1.55 | $ 1.52 | $ 1.42
Diluted (in dollars per share) | $ 0.32 | $ 0.42 | $ 0.42 | $ 0.37 | $ 0.40 | $ 0.42 | $ 0.35 | $ 0.34 | $ 1.54 | $ 1.51 | $ 1.41

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