Note 7 - Investments in Marketable Securities (Details Textual) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2021	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2019
Marketable Securities, Accumulated Unrealized Gain (Loss)		$ 213,661,000		$ 213,661,000		$ 137,593,000
Marketable Securities, Unrealized Gain (Loss), Total		(5,144,000)	$ (77,211,000)	76,068,000	$ (57,680,000)
Proceeds from Sale and Maturity of Debt Securities, Available-for-sale, Total	$ 20,002,000			20,002,000	0
Marketable Securities, Realized Gain (Loss), Total	18,478,000	18,478,000	$ 0	18,478,000	0
Payments to Acquire Marketable Securities, Total	$ 39,995,000			$ 39,995,000	$ (0)
Newly Purchased Marketable Securities [Member]
Marketable Securities, Unrealized Gain (Loss), Total		$ (2,512,000)