Note 11 - Fair Value Measurements - Financial Assets and Liabilities, Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Total Fair Value	$ 259,263	$ 248,953
Assets, fair value	279,231	267,820
Fair Value, Inputs, Level 1 [Member]
Assets, fair value	59,950	60,002
Fair Value, Inputs, Level 2 [Member]
Assets, fair value	216,017	204,577
Fair Value, Inputs, Level 3 [Member]
Assets, fair value	3,264	3,241
Money Market Funds [Member]
Total Fair Value	39,982	41,135
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	39,982	41,135
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value		0
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value		0
Corporate Debt Securities [Member]
Total Fair Value	195,667	170,909
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value		0
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	195,667	170,909
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value		0
US Government Agencies Short-term Debt Securities [Member]
Total Fair Value	20,350	32,068
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value		0
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value	20,350	32,068
US Government Agencies Short-term Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value		0
Auction Rate Securities [Member]
Total Fair Value	3,264	3,241
Auction Rate Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value		0
Auction Rate Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value		0
Auction Rate Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value	3,264	3,241
Certificates of Deposit [Member]
Total Fair Value		1,600
Certificates of Deposit [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value		0
Certificates of Deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value		1,600
Certificates of Deposit [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value		0
Mutual Fund [Member]
Total Fair Value	19,968	18,867
Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Total Fair Value	$ 19,968	18,867
Mutual Fund [Member] | Fair Value, Inputs, Level 2 [Member]
Total Fair Value		0
Mutual Fund [Member] | Fair Value, Inputs, Level 3 [Member]
Total Fair Value		$ 0