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Summary of Significant Accounting Policies (Details) $ / shares in Units, $ in Thousands		6 Months Ended
	Feb. 08, 2021 $ / shares shares	Mar. 31, 2021 USD ($) branch	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Accounting Policies [Line Items]
Number of bank branches | branch		232
Increase (decrease) to retained earnings for the cumulative effect of adopting ASC 326		$ 2,332,953	$ 1,964,612	$ 2,061,767	$ 2,014,133	$ 2,031,127	$ 2,032,995
Cash collateral		1,500			97,600
Loans in process		1,982,225			1,456,072
Proceeds from issuance of preferred stock, net		293,325	0
Dividend rate	4.875%
Public Offering
Accounting Policies [Line Items]
Number of depositary shares | shares	12,000,000
Percentage of preferred stock	2.50%
Public offering price (in usd per share) | $ / shares	$ 25.00
Public Offering | Series A Preferred Stock
Accounting Policies [Line Items]
Number of depositary shares | shares	300,000
Financing Receivable
Accounting Policies [Line Items]
Loans in process		3,348,654			2,738,095
Retained Earnings
Accounting Policies [Line Items]
Increase (decrease) to retained earnings for the cumulative effect of adopting ASC 326		$ 1,468,093	$ 1,384,833	$ 1,443,280	$ 1,420,906	$ 1,365,397	1,335,909
Cumulative Effect, Period of Adoption, Adjustment
Accounting Policies [Line Items]
Increase (decrease) to retained earnings for the cumulative effect of adopting ASC 326							(21,945)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Accounting Policies [Line Items]
Increase (decrease) to retained earnings for the cumulative effect of adopting ASC 326							$ (21,945)

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Carrying amount as of the balance sheet date of cash collateral held for borrowed securities, for which the cash is restricted as to withdrawal or usage.

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