Note 3 - Fair Value of Financial Instruments - Financial Assets Measured on a Recurring Basis (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Money Market Funds [Member]
Money market funds, Fair Value	$ 83,124,000
Fair Value, Recurring [Member]
Assets, Fair Value	103,063,000
Fair Value, Recurring [Member] \| Short-term Investments [Member] \| US Treasury Securities [Member]
Investments, Fair Value Disclosure	4,997,000
Fair Value, Recurring [Member] \| Short-term Investments [Member] \| Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investments, Fair Value Disclosure	8,388,000
Fair Value, Recurring [Member] \| Long-term Investments [Member] \| Corporate Debt Securities [Member]
Investments, Fair Value Disclosure	6,554,000
Fair Value, Recurring [Member] \| Money Market Funds [Member]
Money market funds, Fair Value	83,124,000	$ 60,005,000
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value	83,124,000
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Short-term Investments [Member] \| US Treasury Securities [Member]
Investments, Fair Value Disclosure	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Short-term Investments [Member] \| Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investments, Fair Value Disclosure	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Long-term Investments [Member] \| Corporate Debt Securities [Member]
Investments, Fair Value Disclosure	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Money Market Funds [Member]
Money market funds, Fair Value	83,124,000
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value	19,939,000
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Short-term Investments [Member] \| US Treasury Securities [Member]
Investments, Fair Value Disclosure	4,997,000
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Short-term Investments [Member] \| Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investments, Fair Value Disclosure	8,388,000
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Long-term Investments [Member] \| Corporate Debt Securities [Member]
Investments, Fair Value Disclosure	6,554,000
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Money Market Funds [Member]
Money market funds, Fair Value	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Short-term Investments [Member] \| US Treasury Securities [Member]
Investments, Fair Value Disclosure	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Short-term Investments [Member] \| Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Investments, Fair Value Disclosure	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Long-term Investments [Member] \| Corporate Debt Securities [Member]
Investments, Fair Value Disclosure	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member] \| Money Market Funds [Member]
Money market funds, Fair Value	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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