CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Parenthetical) - $ / shares	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Statement Of Stockholders Equity [Abstract]
Common stock dividends, per share	$ 0.52	$ 0.48
Stock option plan, shares	27,284	28,941
Employee stock purchase plan, shares	8,565	12,471
Restricted stock, shares	47,064	49,648
Treasury Stock, Shares, Retired		512,459
Director stock purchase plan, shares		258