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Note 3 - Fair Value Measurements (Details Textual) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
US Treasury Bill Securities [Member] |  |
Available-for-sale Securities, Total | $ 27.5 | $ 70.0

X
- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_USTreasuryBillSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |