Marketable Securities - General Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Marketable securities sold during period, fair value	$ 2,500	$ 49,400
Marketable securities sold during period, amortized cost basis	2,500	49,500
Net realized gain, (loss)		(100)
Sales of marketable securities	2,492	48,903	$ 1,584
Reclassification unrealized net holding gain, (loss)		$ (100)
Maximum
Schedule of Available-for-sale Securities [Line Items]
Net realized gain, (loss)	(100)
Reclassification unrealized net holding gain, (loss)	$ (100)