Note 6 - Shareholders' Equity - Summary of Certain Additional Information about Stock Options (Details) - Share-Based Payment Arrangement, Option [Member] - USD ($)	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Weighted average grant-date fair value of options granted during the period (in dollars per share)	$ 1.84	$ 3.17
Options in-the-money (in shares) (in shares)	312,661	550,733
Aggregate intrinsic value of options exercised during the period	$ 1.20	$ 5.15

Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration