CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parenthetical) - $ / shares	12 Months Ended
	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