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Note 11 - Stock Repurchase Program (Details Textual) - USD ($)
$ in Thousands
 1 Months Ended 12 Months Ended
Oct. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Aug. 31, 2015
Stock Repurchase Program, Authorized Amount $ 250,000       $ 200,000
Stock Repurchase Program, Period in Force 2 years        
Payments for Repurchase of Common Stock   $ 30,012 $ 149,937 $ 99,942  
Treasury Stock [Member]          
Shares Acquired Under Stock Repurchases Program, Shares   844,500 3,968,706 3,303,500