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Fair Value Measurements - Additional Information (Details) - USD ($)	6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020	Jan. 01, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity securities reclassified	$ 494,089,000		$ 705,838,000
Available-for-sale securities after reclassification	2,392,676,000		2,210,099,000
Available-for-sale securities purchased	522,454,000	$ 159,884,000
Proceeds from sales of available-for-sale securities	0	204,351,000
Held-to-maturity securities purchased	0	0
Proceeds from sale of held-to-maturity securities	$ 0	$ 0
Term of contractual due dates of substantially all mortgage-backed securities (in years)	10 years
Accrued interest for available-for-sale securities	$ 4,128,000		3,285,000
Accrued interest for held-to-maturity securities	1,269,000		$ 1,811,000
Mortgage-backed securities \| Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity securities reclassified				$ (374,680,000)
Available-for-sale securities after reclassification				$ 374,680,000
Consumer loans \| Single-family residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Foreclosed residential real estate properties held as REO	676,000
Real estate acquired through foreclosure	$ 3,134,000