Note 9 - Fair Value Measurements - Summary of Fair Value Measurements by Level (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted cash and cash equivalents	$ 10,676	$ 11,673	$ 8,303
Available-for-sale securities	147,406	172,593
Corporate Debt Securities [Member]
Available-for-sale securities	65,653	67,632
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities	13,158	21,436
Fair Value, Recurring [Member]
Cash and cash equivalents	50,334	43,247
Restricted cash and cash equivalents	10,676	11,673
Total financial assets	360,869	367,736
Fair Value, Recurring [Member] \| Equity Securities [Member]
Available-for-sale securities	152,453	140,223
Fair Value, Recurring [Member] \| Corporate Debt Securities [Member]
Available-for-sale securities	65,653	67,632
Fair Value, Recurring [Member] \| Asset-backed Securities [Member]
Available-for-sale securities	55,185	62,068
Fair Value, Recurring [Member] \| US Treasury Securities [Member]
Available-for-sale securities	13,410	21,457
Fair Value, Recurring [Member] \| US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities	13,158	21,436
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	50,334	43,247
Restricted cash and cash equivalents	10,676	11,673
Total financial assets	277,432	264,521
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Equity Securities [Member]
Available-for-sale securities	152,453	140,223
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Corporate Debt Securities [Member]
Available-for-sale securities	48,584	47,921
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| Asset-backed Securities [Member]
Available-for-sale securities	0	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| US Treasury Securities [Member]
Available-for-sale securities	13,410	21,457
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member] \| US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities	1,975	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents	0	0
Restricted cash and cash equivalents	0	0
Total financial assets	83,437	103,215
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Equity Securities [Member]
Available-for-sale securities	0	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Corporate Debt Securities [Member]
Available-for-sale securities	17,069	19,711
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| Asset-backed Securities [Member]
Available-for-sale securities	55,185	62,068
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| US Treasury Securities [Member]
Available-for-sale securities	0	0
Fair Value, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member] \| US States and Political Subdivisions Debt Securities [Member]
Available-for-sale securities	$ 11,183	$ 21,436

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of 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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