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Note 11 - Stock-based Compensation - Restricted Stock Units (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Non-vested RSU's, beginning balance (in shares) 497,322    
Non-vested RSU's, weighted-average grant-date fair value, beginning balance (in dollars per share) $ 22.46    
Granted RSU's (in shares) 280,590 276,900 337,175
Granted RSU's, weighted-average grant-date fair value (in dollars per share) $ 28.19    
Vested RSU's (in shares) (256,414)    
Vested RSU's, weighted-average grant-date fair value (in dollars per share) $ 23.70    
Forfeited RSU's (in shares) (18,840)    
Forfeited RSU's, weighted-average grant-date fair value (in dollars per share) $ 25.01    
Non-vested RSU's, ending balance (in shares) 502,658 497,322  
Non-vested RSU's, weighted-average grant-date fair value, ending balance (in dollars per share) $ 24.93 $ 22.46  
Vested RSU's but unissued (in shares) 241,924    
Vested RSU's but unissued, weighted-average grant-date fair value (in dollars per share) $ 25.10