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CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parenthetical) - $ / shares | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Increase (Decrease) in Shareholders' Equity [Roll Forward] |  |  |
Distribution of shares of treasury stock for directors' deferred compensation (in shares) | 2,438 | 3,740 | 3,598
Distribution of shares of treasury stock granted for employee restricted stock awards (in shares) | 10,161 | 8,249 | 7,628
Cash dividends declared (in dollars per share) | $ 1.04 | $ 1.04 | $ 1.04
Distribution of shares of treasury stock for directors' compensation (in shares) | 7,880 | 9,532 | 9,673
Distribution of shares of treasury stock for employee compensation (in shares) | 5,861 | 7,661 | 3,303
Sale of shares of treasury stock (in shares) | 14,707 | 15,308 | 16,209
Purchase of shares of treasury stock (in shares) | 1,159 | 612 | 1,184
Restricted stock awards, forfeited (in shares) | 1,139 |  |

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