Note 2 - Investments - Summary of Investments by Investment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale securities, amortized cost	$ 139,448	$ 235,766
Available-for-sale securities, gross unrealized gain	685	15
Available-for-sale securities, gross unrealized loss	(2)	(442)
Available-for-sale securities, fair value	140,131	235,339
US Treasury Securities [Member]
Available-for-sale securities, amortized cost	3,752
Available-for-sale securities, gross unrealized gain	7
Available-for-sale securities, gross unrealized loss	0
Available-for-sale securities, fair value	3,759
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities, amortized cost	6,062	6,751
Available-for-sale securities, gross unrealized gain	57	1
Available-for-sale securities, gross unrealized loss	0	(19)
Available-for-sale securities, fair value	6,119	6,733
Corporate Debt Securities [Member]
Available-for-sale securities, amortized cost	118,859	146,466
Available-for-sale securities, gross unrealized gain	591	14
Available-for-sale securities, gross unrealized loss	(1)	(354)
Available-for-sale securities, fair value	119,449	146,126
Variable Rate Demand Note [Member]
Available-for-sale securities, amortized cost		8,900
Available-for-sale securities, gross unrealized gain		0
Available-for-sale securities, gross unrealized loss		0
Available-for-sale securities, fair value		8,900
Asset-backed Securities [Member]
Available-for-sale securities, amortized cost	4,239	32,986
Available-for-sale securities, gross unrealized gain	29	0
Available-for-sale securities, gross unrealized loss	0	(63)
Available-for-sale securities, fair value	4,268	32,923
Commercial Paper [Member]
Available-for-sale securities, amortized cost	6,464	39,707
Available-for-sale securities, gross unrealized gain	1	0
Available-for-sale securities, gross unrealized loss	(1)	(6)
Available-for-sale securities, fair value	6,464	39,701
Certificates of Deposit [Member]
Available-for-sale securities, amortized cost	72	956
Available-for-sale securities, gross unrealized gain	0	0
Available-for-sale securities, gross unrealized loss	0	0
Available-for-sale securities, fair value	$ 72	$ 956

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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