Summary of Significant Accounting Policies - Narrative (Details)		3 Months Ended		9 Months Ended
	Mar. 01, 2024 USD ($)	Jun. 30, 2024 USD ($) branch	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) branch portfolioSegment classOfPortfolioSegment	Jun. 30, 2023 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)
Accounting Policies [Line Items]
Number of bank branches | branch		210		210
Cash collateral		$ 276,450,000		$ 276,450,000				$ 326,750,000
Fair value of NAV		37,057,000		37,057,000
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		1,094,956,000		$ 1,094,956,000				1,895,940,000
Goodwill		412,213,000		412,213,000		$ 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	$ 105,560,000
Intangible assets	$ 37,022,000
Acquired intangible assets, useful life	6 years