Note 4 - Investments (Details Textual) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
US Treasury Bill Securities [Member]
Available-for-sale Securities, Total	$ 71.2	$ 70.0

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

No definition available.

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant