CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - (Parenthetical) - $ / shares	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Statement Of Stockholders Equity [Abstract]
Common stock dividends, per share	$ 0.54	$ 0.52
Stock option plan, shares	17,927	27,284
Employee stock purchase plan, shares	13,821	8,565
Restricted stock, shares	37,705	47,064