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Note 11 - Derivative Instruments and Hedging Activities (Details Textual)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2017
CAD ($)
Dec. 31, 2016
CAD ($)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ 0 $ 0 $ 400,000    
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months 0        
Canadian Forward Contract, Remaining Maturities of 15 to 17 Months [Member]          
Derivative, Notional Amount $ 2,100,000     $ 2.7  
Derivative, Number of Instruments Held 2     2  
Maximum [Member]          
Maturity Period For Forward Contracts 1 year        
Designated as Hedging Instrument [Member]          
Derivative, Notional Amount $ 2,100,000 3,400,000   $ 2.7 $ 4.5
Not Designated as Hedging Instrument [Member]          
Derivative, Notional Amount $ 200,000 $ 900,000   $ 0.2 $ 1.3