Note 7 - Investments in Marketable Securities - Summary of Investments (Details) - Common Stock [Member] - USD ($)	Mar. 31, 2021	Sep. 30, 2020
Aggregate fair value	$ 293,907,000	$ 179,368,000
Amortized/adjusted cost basis	80,246,000	41,775,000
Pretax net unrealized gains	$ 213,661,000	$ 137,593,000

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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