Note 2 - Significant Accounting Policies - Cash, Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Member]
Amortized cost		$ 56,689	$ 48,315
Gross unrealized gains
Gross unrealized losses
Fair value		56,689	48,315
Cash and Cash Equivalents [Member] | Cash [Member]
Amortized cost		49,819	42,175
Gross unrealized gains
Gross unrealized losses
Fair value		49,819	42,175
Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Amortized cost		6,870	6,140
Gross unrealized gains
Gross unrealized losses
Fair value		6,870	6,140
Cash and Cash Equivalents [Member] | US Treasury Securities Included in Cash and Cash Equivalents [Member]
Amortized cost		39,758
Gross unrealized gains	[1]
Gross unrealized losses		(27)
Fair value		39,731
Short-term Investments [Member]
Amortized cost		79,895	65,493
Gross unrealized gains
Gross unrealized losses		(27)
Fair value		79,868	65,493
Short-term Investments [Member] | Term Bank Deposits [Member]
Amortized cost		40,137	65,493
Gross unrealized gains
Gross unrealized losses
Fair value		$ 40,137	$ 65,493

[1] Represents an amount lower than $1.