Note 1 - Business and Significant Accounting Policies - Shares Used to Calculate Earnings Per Share (Details) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Weighted average number of shares used in computation of basic earnings per share (in shares)	51,017	50,933	50,800	50,709	50,691	50,645	50,623	50,614	50,867	50,643	50,110
Additional shares from the assumed exercise of employee stock options (in shares)									360	326	659
Weighted average number of shares used in computation of diluted earnings per share (in shares)	51,511	51,334	51,094	51,012	50,936	50,963	50,958	51,051	51,227	50,969	50,769