Note 2 - Fair Value of Financial Instruments - Cash and and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents, Fair Value Disclosure	$ 93,027	$ 87,559
Short-term marketable securities, amortization cost	225,196	211,089
Unrealized gains, short-term securities	540	246
Short-term marketable securities, unrealized loss	(2)	(4)
Short-term marketable securities	225,734	211,331
Long-term marketable securities, amortization cost	131,652	118,684
Unrealized gains, long-term securities	1,616	825
Long-term marketable securities, unrealized loss	(8)	(1)
Long-term marketable securities	133,260	119,508
Cash and cash equivalents and marketable securities, amortization cost	449,875	417,332
Unrealized gains, securities	2,156	1,071
Unrealized loss, securities	(10)	(5)
Cash and cash equivalents and short-term and long-term Marketable Securities, fair value	452,021	418,398
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Short-term marketable securities, amortization cost	649	2,239
Unrealized gains, short-term securities	0	0
Short-term marketable securities, unrealized loss	0	0
Short-term marketable securities	649	2,239
Corporate Debt Securities [Member]
Short-term marketable securities, amortization cost	30,508	33,048
Unrealized gains, short-term securities	249	51
Short-term marketable securities, unrealized loss	(1)	(1)
Short-term marketable securities	30,756	33,098
Long-term marketable securities, amortization cost	52,870	32,236
Unrealized gains, long-term securities	920	262
Long-term marketable securities, unrealized loss	(6)	0
Long-term marketable securities	53,784	32,498
Asset-backed Securities [Member]
Short-term marketable securities, amortization cost	5,514	2,438
Unrealized gains, short-term securities	25	11
Short-term marketable securities, unrealized loss	0	0
Short-term marketable securities	5,539	2,449
Long-term marketable securities, amortization cost	40,367	40,001
Unrealized gains, long-term securities	290	193
Long-term marketable securities, unrealized loss	0	(1)
Long-term marketable securities	40,657	40,193
US Government Agencies Debt Securities [Member]
Short-term marketable securities, amortization cost	188,525	173,364
Unrealized gains, short-term securities	266	184
Short-term marketable securities, unrealized loss	(1)	(3)
Short-term marketable securities	188,790	173,545
Long-term marketable securities, amortization cost	37,414	46,447
Unrealized gains, long-term securities	373	370
Long-term marketable securities, unrealized loss	(2)	0
Long-term marketable securities	37,785	46,817
Foreign Exchange Contract [Member]
Long-term marketable securities, amortization cost	1,001
Unrealized gains, long-term securities	33
Long-term marketable securities, unrealized loss	0
Long-term marketable securities	1,034
Amortized Cost [Member]
Cash and Cash Equivalents, Fair Value Disclosure	93,027	87,559
Cash [Member]
Cash and Cash Equivalents, Fair Value Disclosure	33,566	84,102
Cash [Member] | Amortized Cost [Member]
Cash and Cash Equivalents, Fair Value Disclosure	33,566	84,102
Money Market Funds [Member]
Cash and Cash Equivalents, Fair Value Disclosure	58,511	58
Money Market Funds [Member] | Amortized Cost [Member]
Cash and Cash Equivalents, Fair Value Disclosure	58,511	58
Commercial Paper [Member]
Cash and Cash Equivalents, Fair Value Disclosure	950	3,399
Commercial Paper [Member] | Amortized Cost [Member]
Cash and Cash Equivalents, Fair Value Disclosure	$ 950	$ 3,399