Note 5 - Deferred Revenue - Summary of Deferred Service Contract Revenue (Details) - USD ($)		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Balance at beginning of year	[1]	$ 12,566,000	$ 11,656,000
Add: Payments received		17,127,000	14,883,000	[1]
Less: Revenue deferred as of January 1, 2019		(9,451,000)	(8,206,000)	[1]
Less: Revenue included in the beginning balance and recognized as revenue in the current year		(6,021)	(5,767,000)	[1]
Balance at end of year		$ 14,222,000	$ 12,566,000	[1]

[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.