Note 1 - Summary of Significant Accounting Policies - Fair Value of Financial Assets and Liabilities (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets, fair value	$ 34,478	$ 32,650
Money Market Funds [Member]
Assets, fair value	13,971	11,200
Commercial Paper [Member]
Assets, fair value	10,490	21,450
Eurodollar Deposits [Member]
Assets, fair value	10,017
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	13,971	11,200
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets, fair value	13,971	11,200
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	20,507	21,450
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	10,490	21,450
Fair Value, Inputs, Level 2 [Member] | Eurodollar Deposits [Member]
Assets, fair value	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