CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - UNAUDITED (Parenthetical) - $ / shares	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Statement Of Stockholders Equity [Abstract]
Common stock dividends, per share	$ 0.78	$ 0.72
Stock option plan, shares	28,736	34,646
Employee stock purchase plan, shares	13,486	18,767
Restricted stock, shares	47,064	49,648
Treasury Stock, Shares, Retired		512,459
Director stock purchase plan, shares		258