Note 5 - Investments Net Unrealized Gains (Losses) on Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Gain (Loss) on Securities [Line Items]
Net changes in unrealized gains (losses) on investments	$ 9,150	$ (5,503)	$ 23,634	$ (17,245)
Trading Securities
Gain (Loss) on Securities [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	7,608	(5,816)	15,027	(16,755)
Equity securities
Gain (Loss) on Securities [Line Items]
Equity Securities, Change in Unrealized Holding Gain (Loss)	1,404	224	8,600	(702)
Other investments
Gain (Loss) on Securities [Line Items]
Other Investments, Change in Unrealized Holding Gain (Loss)	$ 138	$ 89	$ 7	$ 212