Note 6 - Derivative Instruments and Hedging Activities (Details Textual) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 CAD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 CAD ($)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net, Total	$ 100,000	$ 0	$ 100,000	$ 0
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months	0		$ 0
Maximum [Member]
Maturity Period For Forward Contracts			1 year
Designated as Hedging Instrument [Member]
Derivative, Notional Amount	$ 3,500,000		$ 3,500,000		$ 4.6	$ 2,100,000	$ 2.7
Not Designated as Hedging Instrument [Member]
Derivative, Notional Amount						$ 200,000	$ 0.2