SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)			12 Months Ended
	Mar. 01, 2024 USD ($)	Feb. 08, 2021 USD ($) $ / shares shares	Sep. 30, 2025 USD ($) portfolioSegment segment payment classOfPortfolioSegment branch	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Oct. 27, 2020 USD ($)	Oct. 09, 2013 USD ($)
Summary of Significant Accounting Policies [Line Items]
Number of offices | branch			208
Dividend rate		4.875%
Proceeds from issuance of preferred stock, net		$ 293,325,000
Cash collateral			$ 118,400,000	$ 168,200,000
Fair value of NAV			35,564,000
Remaining unfunded commitment			$ 13,388,000
Days past grace period to initiate contact to cure deficiency			30 days
Minimum days past due to begin foreclosure proceedings			90 days
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
Minimum days past due to stop accrual of interest			90 days
Number of payments required | payment			6
Number of operating segments | segment			1
Number of reportable segments | segment			1
Goodwill			$ 414,722,000	411,360,000	$ 304,750,000
Intangible asset impairment			$ 0	$ 0	$ 0
Weighted average remaining contractual term, exercisable			5 years
Civil money penalty from consent order issued						$ 200,000	$ 34,000
Minimum
Summary of Significant Accounting Policies [Line Items]
Award vesting period (in years)			3 years
Minimum | Stock Options and Restricted Stock
Summary of Significant Accounting Policies [Line Items]
Award vesting period (in years)			2 years
Maximum
Summary of Significant Accounting Policies [Line Items]
Award vesting period (in years)			5 years
Weighted average remaining contractual term, exercisable			10 years
Maximum | Stock Options and Restricted Stock
Summary of Significant Accounting Policies [Line Items]
Award vesting period (in years)			5 years
Luther Burbank
Summary of Significant Accounting Policies [Line Items]
Goodwill	$ 107,890,000
Intangible assets	$ 37,022,000
Acquired intangible assets, useful life	6 years
Public Offering
Summary of Significant Accounting Policies [Line Items]
Number of shares issued in transaction (in shares) | shares		12,000,000
Percentage of preferred stock		2.50%
Public offering price (in dollars per share) | $ / shares		$ 25.00
Series A Preferred Stock | Public Offering
Summary of Significant Accounting Policies [Line Items]
Number of shares issued in transaction (in shares) | shares		300,000