Consolidated Statements of Changes in Stockholders' Equity (Parentheticals) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Additional Paid-in Capital [Member]
Common shares released from Employee Benefit Trust, shares (in shares)	118,371	142,065	147,616
Restricted stock unit awards vested, shares (in shares)	284,595	245,311	204,310
Exercise of stock options, shares (in shares)	4,400	103,530	45,785
Common shares released from Employee Benefit Trust, shares (in shares)	118,371	142,065	147,616
Restricted stock unit awards vested, shares (in shares)	284,595	245,311	204,310
Exercise of stock options, shares (in shares)	4,400	103,530	45,785
Treasury Stock [Member] | Shares Purchased to Fund Options Exercised [Member]
Purchase of treasury shares (in shares)	241,625	403,695	735,599
Purchase of treasury shares (in shares)	241,625	403,695	735,599
Treasury Stock [Member] | Restricted Stock Awards Repurchased to Satisfy Tax Obligations [Member]
Purchase of treasury shares (in shares)	90,779	85,982	65,666
Purchase of treasury shares (in shares)	90,779	85,982	65,666
Retained Earnings [Member]
Dividends on common stock (in dollars per share)	$ 0.72	$ 0.68	$ 0.64
Dividends on common stock (in dollars per share)	$ 0.72	$ 0.68	$ 0.64
Exercise of stock options, shares (in shares)	4,400
Purchase of treasury shares (in shares)	241,625	403,695
Purchase of treasury shares (in shares)	241,625	403,695
Exercise of stock options, shares (in shares)	4,400