Note 4 - Investments - Summary of Investments by Investment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale securities, amortized cost	$ 235,766	$ 241,977
Available-for-sale securities, gross unrealized gain	15	50
Available-for-sale securities, gross unrealized loss	(442)	(290)
Available-for-sale securities, fair value	235,339	241,737
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities, amortized cost	6,751	27,725
Available-for-sale securities, gross unrealized gain	1
Available-for-sale securities, gross unrealized loss	(19)	(68)
Available-for-sale securities, fair value	6,733	27,657
Corporate Debt Securities [Member]
Available-for-sale securities, amortized cost	146,466	146,549
Available-for-sale securities, gross unrealized gain	14	49
Available-for-sale securities, gross unrealized loss	(354)	(197)
Available-for-sale securities, fair value	146,126	146,401
Variable Rate Demand Note [Member]
Available-for-sale securities, amortized cost	8,900	3,500
Available-for-sale securities, fair value	8,900	3,500
Asset-backed Securities [Member]
Available-for-sale securities, amortized cost	32,986	7,197
Available-for-sale securities, gross unrealized loss	(63)	(12)
Available-for-sale securities, fair value	32,923	7,185
Commercial Paper [Member]
Available-for-sale securities, amortized cost	39,707	57,006
Available-for-sale securities, gross unrealized gain		1
Available-for-sale securities, gross unrealized loss	(6)	(13)
Available-for-sale securities, fair value	39,701	$ 56,994
Certificates of Deposit [Member]
Available-for-sale securities, amortized cost	956
Available-for-sale securities, fair value	$ 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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Amount, before tax, of unrealized loss 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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