CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Stockholders Equity [Abstract]
Common stock dividends, per share	$ 1.04	$ 0.98	$ 0.9
Stock option plan, shares	30,567	44,067	39,787
Employee stock purchase plan, shares	17,578	23,779	25,136
Restricted stock, shares	47,064	49,468	55,784
Treasury Stock, Shares, Retired	0	512,459	870,450
Director stock purchase plan, shares	258	837