ALLOWANCE FOR LOAN LOSSES - Gross Loans by Payment Activity (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 20,288,338	$ 21,120,107
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	11,851,657	12,205,420
Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	4,631,321	4,556,200
Commercial loans \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	3,588,950	3,732,155
Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	2,386,363	2,332,732
Commercial loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	1,105,101	1,424,016
Commercial loans \| Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	139,922	160,317
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	8,436,681	8,914,687
Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	78,243	182,415
Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	7,936,931	8,280,300
Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	88,696	108,060
Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	271,286	269,857
Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	61,525	74,055
Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 20,159,710	$ 21,050,566
% of Total Loans	99.40%	99.70%
Performing Loans \| Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 11,748,117	$ 12,159,121
% of Total Loans	99.10%	99.60%
Performing Loans \| Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 4,612,200	$ 4,537,457
% of Total Loans	99.60%	99.60%
Performing Loans \| Commercial loans \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 3,518,978	$ 3,705,793
% of Total Loans	98.10%	99.30%
Performing Loans \| Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 2,375,316	$ 2,332,732
% of Total Loans	99.50%	100.00%
Performing Loans \| Commercial loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,101,701	$ 1,422,896
% of Total Loans	99.70%	99.90%
Performing Loans \| Commercial loans \| Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 139,922	$ 160,243
% of Total Loans	100.00%	100.00%
Performing Loans \| Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 8,411,593	$ 8,891,445
% of Total Loans	99.70%	99.70%
Performing Loans \| Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 77,483	$ 181,567
% of Total Loans	99.00%	99.50%
Performing Loans \| Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 7,913,190	$ 8,258,812
% of Total Loans	99.70%	99.70%
Performing Loans \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 88,673	$ 108,060
% of Total Loans	100.00%	100.00%
Performing Loans \| Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 270,874	$ 269,261
% of Total Loans	99.80%	99.80%
Performing Loans \| Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 61,373	$ 73,745
% of Total Loans	99.80%	99.60%
Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 128,628	$ 69,541
% of Total Loans	0.60%	0.30%
Non-Performing Loans \| Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 103,540	$ 46,299
% of Total Loans	0.90%	0.40%
Non-Performing Loans \| Commercial loans \| Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 19,121	$ 18,743
% of Total Loans	0.40%	0.40%
Non-Performing Loans \| Commercial loans \| Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 69,972	$ 26,362
% of Total Loans	1.90%	0.70%
Non-Performing Loans \| Commercial loans \| Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 11,047	$ 0
% of Total Loans	0.50%	0.00%
Non-Performing Loans \| Commercial loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 3,400	$ 1,120
% of Total Loans	0.30%	0.10%
Non-Performing Loans \| Commercial loans \| Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 0	$ 74
% of Total Loans	0.00%	0.00%
Non-Performing Loans \| Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 25,088	$ 23,242
% of Total Loans	0.30%	0.30%
Non-Performing Loans \| Consumer loans \| Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 760	$ 848
% of Total Loans	1.00%	0.50%
Non-Performing Loans \| Consumer loans \| Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 23,741	$ 21,488
% of Total Loans	0.30%	0.30%
Non-Performing Loans \| Consumer loans \| Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 23	$ 0
% of Total Loans	0.00%	0.00%
Non-Performing Loans \| Consumer loans \| HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 412	$ 596
% of Total Loans	0.20%	0.20%
Non-Performing Loans \| Consumer loans \| Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 152	$ 310
% of Total Loans	0.20%	0.40%