Note 1 - General - Cash, Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Member]
Amortized cost	$ 70,778	$ 56,689
Gross unrealized gains
Gross unrealized losses
Fair value	70,778	56,689
Short-term investments
Amortized cost	70,778	56,689
Gross unrealized gains
Gross unrealized losses
Fair value	70,778	56,689
Cash and Cash Equivalents [Member] | Cash [Member]
Amortized cost	61,749	49,819
Gross unrealized gains
Gross unrealized losses
Fair value	61,749	49,819
Short-term investments
Amortized cost	61,749	49,819
Gross unrealized gains
Gross unrealized losses
Fair value	61,749	49,819
Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Amortized cost	9,029	6,870
Gross unrealized gains
Gross unrealized losses
Fair value	9,029	6,870
Short-term investments
Amortized cost	9,029	6,870
Gross unrealized gains
Gross unrealized losses
Fair value	9,029	6,870
Cash and Cash Equivalents [Member] | US Treasury Securities Included in Cash and Cash Equivalents [Member]
Amortized cost	37,751	39,758
Gross unrealized gains
Gross unrealized losses	(34)	(27)
Fair value	37,717	39,731
Short-term investments
Amortized cost	37,751	39,758
Gross unrealized gains
Gross unrealized losses	(34)	(27)
Fair value	37,717	39,731
Short-term Investments [Member]
Amortized cost	82,926	79,895
Gross unrealized gains
Gross unrealized losses	(34)	(27)
Fair value	82,892	79,868
Short-term investments
Amortized cost	82,926	79,895
Gross unrealized gains
Gross unrealized losses	(34)	(27)
Fair value	82,892	79,868
Short-term Investments [Member] | Term Bank Deposits [Member]
Amortized cost	45,175	40,137
Gross unrealized gains
Gross unrealized losses
Fair value	45,175	40,137
Short-term investments
Amortized cost	45,175	40,137
Gross unrealized gains
Gross unrealized losses
Fair value	$ 45,175	$ 40,137