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Summary of Significant Accounting Policies - Narrative (Details)			3 Months Ended		6 Months Ended
	Mar. 01, 2024 USD ($)	Feb. 08, 2021 USD ($) $ / shares shares	Mar. 31, 2024 USD ($) branch	Mar. 31, 2023 USD ($)	Mar. 31, 2024 USD ($) portfolioSegment classOfPortfolioSegment branch	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)
Accounting Policies [Line Items]
Number of bank branches | branch			210		210
Dividend rate		4.875%
Proceeds from issuance of preferred stock, net		$ 293,325,000
Cash collateral			$ 282,700,000		$ 282,700,000			$ 326,750,000
Fair value of NAV			36,703,000		36,703,000
Remaining unfunded commitment			3,280,000		$ 3,280,000
Number of portfolio segments for loans held for investment | portfolioSegment					2
Number of classes disaggregating the commercial loan portfolio | classOfPortfolioSegment					5
Number of classes disaggregating the consumer loan portfolio | classOfPortfolioSegment					5
Loans in process			1,303,978,000		$ 1,303,978,000			1,895,940,000
Goodwill			411,401,000		411,401,000		$ 305,125,000	304,750,000
Goodwill and intangible asset impairment			0	$ 0	0	$ 0
Luther Burbank
Accounting Policies [Line Items]
Goodwill	$ 105,836,000
Intangible assets	$ 37,022,000
Acquired intangible assets, useful life	6 years
Financing Receivable
Accounting Policies [Line Items]
Loans in process			$ 3,859,416,000		$ 3,859,416,000			$ 3,625,333,000
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
Series A Preferred Stock | Public Offering
Accounting Policies [Line Items]
Number of depositary shares | shares		300,000

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