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Investment Securities - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Available-for-sale securities purchased	$ 530,227	$ 1,064,815	$ 358,709
Payments to acquire held-to-maturity securities	0
Proceeds from sales of available-for-sale investment securities	1,499	204,351	$ 491
Proceeds from sale of held-to-maturity securities	0
Accrued interest for available-for-sale securities	3,459	3,285
Accrued interest for held-to-maturity securities	$ 944	1,811
Term of contractual due dates (in years)	25 years
Accounting Standards Update 2019-04
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Held-to-maturity, at amortized costs		(374,680)
Available-for-sale, at amortized cost		$ 374,680

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