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Term of Contractual Due Dates of Substantially All Mortgage-Backed Securities

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Fair Value Measurements - Additional Information (Details) - USD ($)	6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jan. 01, 2020	Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt Securities, Held-to-maturity	$ 920,255,000			$ 1,443,480,000
Available-for-sale, at amortized cost	1,667,467,000			$ 1,458,071,000
Available-for-sale securities purchased	159,884,000	$ 290,574,000
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
Accounting Standards Update 2019-04 \| Mortgage-backed securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt Securities, Held-to-maturity			$ (374,680,000)
Available-for-sale, at amortized cost			$ 374,680,000