CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parentheticals) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Stockholders' Equity [Abstract]
Issuance of common stock to non-employee directors	5,922	4,410	4,620
Exercise of stock options	26,500	500	34,000
Dividends paid (in dollars per share)	$ 0.72	$ 0.68	$ 0.64