Fair Value Measurements and Disclosures about Fair Value of Financial Instruments - Summary of Aggregate Fair Value and the Aggregate Remaining Principal Balance for Residential Mortgage Loans Held for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Fair Value Measurements and Disclosures about Fair Value of Financial Instruments
Residential mortgage loans held for sale Aggregate Fair Value	$ 147,248	$ 208,493
Residential mortgage loans held for sale Aggregate Principal Balance	144,877	198,138
Residential mortgage loans held for sale Difference	$ 2,371	$ 10,355