Note 6 - Marketable Securities - Amortized Cost and Unrealized Gain (Loss) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized cost	$ 114,480	$ 114,531
Unrealized gains, net	653,509	592,423
Marketable securities	$ 767,989	$ 706,954

Amount of investment in marketable security.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Name:	us-gaap_MarketableSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant