Investments in Unconsolidated Entities - Outstanding Borrowings of Unconsolidated Entities and Equity in Income (Loss) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019
Schedule of Equity Method Investments [Line Items]
Total equity of unconsolidated entities	$ 673,605	$ 633,767	$ 593,171	$ 564,316	$ 547,482	$ 538,754
Total outstanding borrowings of unconsolidated entities	1,110,053		1,130,801
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated entities	4,361		4,003
Total equity of unconsolidated entities	8,905		7,079
Total outstanding borrowings of unconsolidated entities	$ 12,478		$ 8,807