Fair Value Measurements - Mortgage Loans Held for Sale (Details) - Fair value, measurements, nonrecurring - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other repossessed property	$ 17,991	$ 34,721
Impaired loans	669,968	413,644
Mortgage loans held for sale, at lower of cost or fair value	12,371	7,351
Property held for sale	600	2,757
Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other repossessed property	0	0
Impaired loans	0	0
Mortgage loans held for sale, at lower of cost or fair value	0	0
Property held for sale	0	0
Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other repossessed property	0	0
Impaired loans	0	0
Mortgage loans held for sale, at lower of cost or fair value	12,371	7,351
Property held for sale	0	0
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Other repossessed property	17,991	34,721
Impaired loans	669,968	413,644
Mortgage loans held for sale, at lower of cost or fair value	0	0
Property held for sale	$ 600	$ 2,757