CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Parenthetical) - $ / shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement Of Stockholders Equity [Abstract]
Common stock dividends, per share	$ 0.26	$ 0.24
Stock option plan, shares	17,362	21,694
Employee stock purchase plan, shares	4,431	6,937
Restricted stock, shares	7,288	15,162
Treasury Stock, Shares, Retired		512,459