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CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parenthetical) - $ / shares
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Increase (Decrease) in Shareholders' Equity [Roll Forward]      
Distribution of shares of treasury stock for directors' deferred compensation (in shares) 3,740 3,598 3,467
Distribution of shares of treasury stock granted for employee restricted stock awards (in shares) 8,249 7,628 11,279
Cash dividends declared (in dollars per share) $ 1.04 $ 1.04 $ 1.04
Distribution of shares of treasury stock for directors' compensation (in shares) 9,532 9,673 8,385
Distribution of shares of treasury stock for employee compensation (in shares) 7,661 3,303 3,595
Sale of shares of treasury stock (in shares) 15,308 16,209  
Purchase of shares of treasury stock (in shares) 612 1,184