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NET INCOME PER COMMON SHARE AND EQUITY PER COMMON SHARE (Details) - $ / shares | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Shares used in computing basic and diluted net income per share [Abstract] |  |  |  |  |
Weighted average common shares outstanding during the period for Basic computation (in shares) | 9,237,000 | 9,163,000 | 9,230,000 | 9,149,000 |
Dilutive effect of employee stock options and awards (in shares) | 56,000 | 72,000 | 57,000 | 71,000 |
Weighted average common shares outstanding during the period for Diluted computation (in shares) | 9,293,000 | 9,235,000 | 9,287,000 | 9,220,000 |
Class A Stock [Member] |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |
Common stock, shares authorized (in shares) | 15,000,000 |  | 15,000,000 |  |
Common stock, shares issued (in shares) | 8,359,738 | 8,306,073 | 8,359,738 | 8,306,073 |
Common stock, shares outstanding (in shares) | 8,359,738 | 8,306,073 | 8,359,738 | 8,306,073 |
Common stock, par value (in dollars per share) | $ 1.00 |  | $ 1.00 |  |
Common stock, shares issued during period (in shares) | 11,495 | 42,275 | 26,284 | 60,040 |
Earnings Per Share [Abstract] |  |  |  |  |
Equity per common share (in dollars per share) |  |  | $ 16.05 |  | $ 15.98
Class B Stock [Member] |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |
Common stock, shares authorized (in shares) | 1,040,000 |  | 1,040,000 |  |
Common stock, shares issued (in shares) | 881,452 | 881,452 | 881,452 | 881,452 |
Common stock, shares outstanding (in shares) | 881,452 | 881,452 | 881,452 | 881,452 |
Common stock, par value (in dollars per share) | $ 1.00 |  | $ 1.00 |  |
Stock Option [Member] |  |  |  |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract] |  |  |  |  |
Shares excluded from calculations of diluted net income per share (in shares) | 0 | 0 | 0 | 0 |
Restricted Stock [Member] |  |  |  |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share [Abstract] |  |  |  |  |
Shares excluded from calculations of diluted net income per share (in shares) | 5,000 | 1,400 | 5,000 | 3,400 |

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