Note 3 - Fair Value Measurement - Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets, Fair Value	$ 149,040
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value	26,024
Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value	123,016
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value
Fair Value, Recurring [Member]
Assets, Fair Value		$ 169,229
Fair Value, Recurring [Member] | Reverse Repurchase Agreements [Member]
Assets, Fair Value	123,016	154,258
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value		14,971
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		154,258
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Reverse Repurchase Agreements [Member]
Assets, Fair Value	123,016	154,258
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	26,024	14,971
Fair Value, Recurring [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value	26,024	14,971
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