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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted average number of shares used in computation of basic earnings per share (in shares)	50,691	50,645	50,623	50,614	50,607	50,367	49,771	49,680	50,643	50,110	49,469
Additional shares from the assumed exercise of employee stock options (in shares)									326	659	671
Weighted average number of shares used in computation of diluted earnings per share (in shares)	50,936	50,963	50,958	51,051	51,053	50,867	50,624	50,491	50,969	50,769	50,140