Note 2 - Investments (Tables)

12 Months Ended

Dec. 31, 2019

Notes Tables

Available-for-sale Securities [Table Text Block]

	December 31, 2019
	Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Available-for-sale securities:
U.S. Treasury securities	$	3,752	$	7	$	—		$	3,759
Municipal bonds		6,062		57		—			6,119
Corporate notes/bonds		118,859		591		(1	)		119,449
Asset-backed securities		4,239		29		—			4,268
Commercial paper		6,464		1		(1	)		6,464
Certificate of deposit		72		—		—			72
Total investments	$	139,448	$	685	$	(2	)	$	140,131

	December 31, 2018
	Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Available-for-sale securities:
Municipal bonds	$	6,751	$	1	$	(19	)	$	6,733
Corporate notes/bonds		146,466		14		(354	)		146,126
Variable rate demand notes		8,900		—		—			8,900
Asset-backed securities		32,986		—		(63	)		32,923
Commercial paper		39,707		—		(6	)		39,701
Certificate of deposit		956		—		—			956
Total investments	$	235,766	$	15	$	(442	)	$	235,339

Investments Classified by Contractual Maturity Date [Table Text Block]

	Cost		Fair Value
Due in one year or less	$	88,803	$	89,049
Due between one and five years		50,645		51,082
	$	139,448	$	140,131