Note 6 - Investments - Net Changes in Unrealized Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Gain (Loss) on Securities [Line Items]
Net changes in unrealized gains (losses) on investments	$ 3,132	$ 4,742	$ 1,984	$ 28,068
Trading Securities
Gain (Loss) on Securities [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	(1,150)	4,132	9,990	18,962
Equity securities
Gain (Loss) on Securities [Line Items]
Equity Securities, Change in Unrealized Holding Gain (Loss)	4,455	563	(8,158)	9,170
Other investments
Gain (Loss) on Securities [Line Items]
Other Investments, Change in Unrealized Holding Gain (Loss)	$ (173)	$ 47	$ 152	$ (64)