Marketable Securities - General Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Marketable securities sold during period, fair value	$ 0	$ 0	$ 49,400	$ 0
Marketable securities sold during period, amortized cost basis	$ 0	$ 0	49,500	$ 0
Net realized losses			100
Sales of marketable securities			48,904
Reclassification unrealized net holding losses			$ 100