Fair Value Measurements - Additional Information (Details) - USD ($)	9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jan. 01, 2020	Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity securities, at amortized cost	$ 827,316,000			$ 1,443,480,000
Available-for-sale, at amortized cost	2,022,414,000			1,458,071,000
Available-for-sale securities purchased	684,292,000	$ 327,670,000
Proceeds from sales of available-for-sale securities	204,351,000	491,000
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	$ 4,154,000			3,712,000
Accrued interest for held-to-maturity securities	2,126,000			$ 3,716,000
Mortgage-backed securities | Accounting Standards Update 2019-04
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity securities, at amortized cost			$ (374,680,000)
Available-for-sale, at amortized cost			$ 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	1,346,000
Real estate acquired through foreclosure	$ 4,104,000