Note 4 - Fair Value Measurements - Fair Value of Assets and Liabilities Measured On Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Reported Value Measurement [Member]
Cash and cash equivalents	$ 64,211	$ 15,601
Restricted cash equivalents	25
Ownership interests	10,792
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents	64,211	15,601
Restricted cash equivalents	25
Ownership interests	10,792
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents	0	0
Restricted cash equivalents	0
Ownership interests	0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Cash and cash equivalents	0	$ 0
Restricted cash equivalents	0
Ownership interests	$ 0