Note 5 - Common Shares and Earnings Per Share - Reconciliation of Weighted Average Common Shares Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted Average Common Shares Outstanding – Basic (in shares)	39,457,261	38,546,459	37,494,986
Plus Outstanding Share Awards net of Share Reductions for Unrecognized Stock-Based Compensation Expense and Excess Tax Benefits:
Shares Expected to be Awarded for Stock Performance Awards Granted to Executive Officers based on Measurement Period-to-Date Performance (in shares)	210,784	118,644	100,194
Underlying Shares Related to Nonvested Restricted Stock Units Granted to Employees (in shares)	56,952	45,712	36,180
Nonvested Restricted Shares (in shares)	20,380	16,778	22,848
Shares Expected to be Issued Under the Deferred Compensation Program for Directors (in shares)	2,970	3,417	13,488
Potentially Dilutive Stock Options (in shares)			330
Total Dilutive Shares (in shares)	291,086	184,551	173,040
Weighted Average Common Shares Outstanding – Diluted (in shares)	39,748,347	38,731,010	37,668,026