ALLOWANCE FOR LOAN LOSSES - Loans Based on Credit Quality Indicators (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 20,288,338	$ 21,120,107
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 19,406,000	$ 20,615,520
Pass \| Accounts Receivable \| Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	95.70%	97.60%
Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 247,830	$ 77,159
Special Mention \| Accounts Receivable \| Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	1.20%	0.40%
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 629,353	$ 427,428
Substandard \| Accounts Receivable \| Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	3.10%	2.00%
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 5,131	$ 0
Doubtful \| Accounts Receivable \| Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.00%	0.00%
Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 24	$ 0
Loss \| Accounts Receivable \| Credit Concentration Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Total grade as a % of total loans	0.00%	0.00%
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 11,851,657	$ 12,205,420
Commercial loans \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	10,994,504	11,723,996
Commercial loans \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	247,830	77,159
Commercial loans \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	604,168	404,265
Commercial loans \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	5,131	0
Commercial loans \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	24	0
Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	4,631,321	4,556,200
Commercial loans \| Multi-family \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	4,337,347	4,476,283
Commercial loans \| Multi-family \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	136,817	12,377
Commercial loans \| Multi-family \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	154,807	67,540
Commercial loans \| Multi-family \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,350	0
Commercial loans \| Multi-family \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	3,588,950	3,732,155
Commercial loans \| Commercial real estate \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	3,339,062	3,587,682
Commercial loans \| Commercial real estate \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	61,580	40,297
Commercial loans \| Commercial real estate \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	185,527	104,176
Commercial loans \| Commercial real estate \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,781	0
Commercial loans \| Commercial real estate \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,386,363	2,332,732
Commercial loans \| Commercial & industrial \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	2,086,750	2,107,780
Commercial loans \| Commercial & industrial \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	44,998	21,264
Commercial loans \| Commercial & industrial \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	254,591	203,688
Commercial loans \| Commercial & industrial \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans \| Commercial & industrial \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	24	0
Commercial loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,105,101	1,424,016
Commercial loans \| Construction \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,091,682	1,392,332
Commercial loans \| Construction \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	4,435	3,221
Commercial loans \| Construction \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	8,984	28,463
Commercial loans \| Construction \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans \| Construction \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans \| Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	139,922	160,317
Commercial loans \| Land - acquisition & development \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	139,663	159,919
Commercial loans \| Land - acquisition & development \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans \| Land - acquisition & development \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	259	398
Commercial loans \| Land - acquisition & development \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial loans \| Land - acquisition & development \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	8,436,681	8,914,687
Consumer loans \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	8,411,496	8,891,524
Consumer loans \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	25,185	23,163
Consumer loans \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	78,243	182,415
Consumer loans \| Construction \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	77,483	181,567
Consumer loans \| Construction \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Construction \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	760	848
Consumer loans \| Construction \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Construction \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	7,936,931	8,280,300
Consumer loans \| Single-family residential \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	7,913,120	8,258,812
Consumer loans \| Single-family residential \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Single-family residential \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	23,811	21,488
Consumer loans \| Single-family residential \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Single-family residential \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	88,696	108,060
Consumer loans \| Land - consumer lot loans \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	88,613	108,060
Consumer loans \| Land - consumer lot loans \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Land - consumer lot loans \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	83	0
Consumer loans \| Land - consumer lot loans \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Land - consumer lot loans \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	271,286	269,857
Consumer loans \| HELOC \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	270,874	269,261
Consumer loans \| HELOC \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| HELOC \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	412	596
Consumer loans \| HELOC \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| HELOC \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	61,525	74,055
Consumer loans \| Consumer \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	61,406	73,824
Consumer loans \| Consumer \| Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Consumer \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	119	231
Consumer loans \| Consumer \| Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer loans \| Consumer \| Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 0	$ 0