Note 4 - Investments (Details Textual) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax, Total	$ 0	$ 0
Certificates of Deposit [Member]
Available-for-sale Securities, Total	$ 67,200,000	$ 24,000,000

Amount before tax of unrealized gain (loss) in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

No definition available.

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant