Note 1 - Summary of Significant Accounting Policies - Fair Value of Financial Assets and Liabilities (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets, fair value	$ 35,374	$ 34,478
Money Market Funds [Member]
Assets, fair value	13,160	13,971
Commercial Paper [Member]
Assets, fair value	12,188	10,490
Eurodollar Deposits [Member]
Assets, fair value	10,026	10,017
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	13,160	13,971
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets, fair value	13,160	13,971
Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Assets, fair value
Fair Value, Inputs, Level 1 [Member] | Eurodollar Deposits [Member]
Assets, fair value
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	22,214	20,507
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Assets, fair value
Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Assets, fair value	12,188	10,490
Fair Value, Inputs, Level 2 [Member] | Eurodollar Deposits [Member]
Assets, fair value	10,026	10,017
Fair Value, Inputs, Level 3 [Member]
Assets, fair value
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Assets, fair value
Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member]
Assets, fair value
Fair Value, Inputs, Level 3 [Member] | Eurodollar Deposits [Member]
Assets, fair value