Note 5 - Deferred Revenue (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017		Jan. 01, 2018
Revenue, Remaining Performance Obligation, Percentage	76.00%
Revenue, Remaining Performance Obligation, Amount	$ 14,200,000
Contract with Customer, Liability, Total	14,222,000	$ 12,566,000	[1]	$ 11,656,000	[1]	$ 11,656,000
Deferred Revenue Costs Incurred	9,300,000	7,800,000		$ 6,000,000
Extended Product Warranty Accrual, Current	$ 1,999,000	$ 3,159,000

[1] The 2018 presentation in the table above has been revised to include deferred revenue of $11,656 at January 1, 2018 and $12,566 at December 31, 2018 and related changes therein that were previously excluded.