Note 7 - Shareholders' Equity - Summary of Certain Additional Information about Stock Options (Details) - Share-based Payment Arrangement, Option [Member] - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Weighted average grant-date fair value of options granted during the period (in dollars per share)	$ 3.17	$ 1.02
Options in-the-money (in shares) (in shares)	550,733
Aggregate intrinsic value of options exercised during the period	$ 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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