Note 3 - Fair Value Measurement - Assets Measured on Recurring Basis (Details) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets, Fair Value	$ 149,930	$ 169,229
Reverse Repurchase Agreements [Member]
Assets, Fair Value	137,522	154,258
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value	12,408	14,971
Fair Value, Inputs, Level 1 [Member] | Reverse Repurchase Agreements [Member]
Assets, Fair Value	0	0
Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value	137,522	154,258
Fair Value, Inputs, Level 2 [Member] | Reverse Repurchase Agreements [Member]
Assets, Fair Value	137,522	154,258
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Reverse Repurchase Agreements [Member]
Assets, Fair Value	0	0
Money Market Funds [Member]
Assets, Fair Value	12,408	14,971
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value	12,408	14,971
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value	0	0
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value	$ 0	$ 0