Fair Value Measurements - Valuation Techniques and Significant Unobservable Inputs Used for Level 3 Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Property held for sale	$ 600	$ 2,757
Fair value, measurements, nonrecurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other repossessed property	17,991	34,721
Impaired loans	669,968	413,644
Fair value, measurements, nonrecurring | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other repossessed property	17,991	34,721
Impaired loans	669,968	$ 413,644
Property held for sale	$ 600