Note 3 - Fair value Measurement - Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets, Fair Value	$ 169,229
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value	14,971
Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value	154,258
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value
Fair Value, Recurring [Member]
Assets, Fair Value		$ 184,969
Fair Value, Recurring [Member] | Reverse Repurchase Agreements [Member]
Assets, Fair Value	154,258	165,157
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value		19,812
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Reverse Repurchase Agreements [Member]
Assets, Fair Value
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value		165,157
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Reverse Repurchase Agreements [Member]
Assets, Fair Value	154,258	165,157
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Reverse Repurchase Agreements [Member]
Assets, Fair Value
Fair Value, Recurring [Member] | Money Market Funds [Member]
Assets, Fair Value	14,971	19,812
Fair Value, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value	14,971	19,812
Fair Value, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value
Fair Value, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value