Marketable securities (Details) - USD ($)	Dec. 31, 2020	Sep. 30, 2020
Cost Basis [Member]
Other Investment Not Readily Marketable [Line Items]
Fixed income mutual funds	$ 11,255,000	$ 12,313,000
Corporate bonds and notes	4,500,000	5,357,000
Other Investments	19,289,000	17,670,000
Other Investments	(19,289,000)	(17,670,000)
Certificates of deposit	3,534,000
Gross Unrealized Gains [Member]
Other Investment Not Readily Marketable [Line Items]
Fixed income mutual funds	80,000	3,000
Corporate bonds and notes	18,000
Other Investments	105,000	3,000
Other Investments	(105,000)	(3,000)
Certificates of deposit	7,000
Fair Value [Member]
Other Investment Not Readily Marketable [Line Items]
Fixed income mutual funds	11,335,000	12,316,000
Corporate bonds and notes	4,490,000	5,351,000
Other Investments	19,366,000	17,667,000
Other Investments	(19,366,000)	(17,667,000)
Certificates of deposit	3,541,000
Gross Unrealized Losses [Member]
Other Investment Not Readily Marketable [Line Items]
Corporate bonds and notes	(28,000)	(6,000)
Other Investments	28,000	6,000
Other Investments	$ (28,000)	$ (6,000)