Note 5 - Fair Value Measurements (Details Textual) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents, Noncurrent, Total	$ 603,000	$ 603,000
Overnight Reverse Repurchase Agreements [Member]
Unrealized Gain (Loss) on Investments, Total	0	0
Money Market Funds [Member] \| Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure	343,400,000	57,600,000
Overnight Reverse Repurchase Agreements [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash and Cash Equivalents, Fair Value Disclosure	0	100,000,000.0
Collateral for Irrevocable Standby Letter of Credit [Member] \| Fair Value, Inputs, Level 1 [Member]
Restricted Cash and Cash Equivalents, Noncurrent, Total	$ 603,000	$ 603,000

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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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.

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Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as noncurrent. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, 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.

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Amount of unrealized gain (loss) on investment.

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