Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Sep. 30, 2022
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 17,170,385	$ 16,286,372
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	10,358,994	9,728,501
Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	2,695,759	2,626,479
Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	3,213,308	3,111,112
Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	2,621,266	2,343,403
Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	1,637,499	1,423,891
Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	191,162	223,616
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	6,811,391	6,557,871
Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	408,563	397,343
Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	5,967,678	5,726,979
Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	147,078	151,945
Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	214,904	206,033
Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	73,168	75,571
Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 17,141,205	$ 16,251,838
% of Total Loans	99.80%	99.80%
Performing Loans | Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 10,347,574	$ 9,712,205
% of Total Loans	99.90%	99.80%
Performing Loans | Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 2,689,880	$ 2,620,567
% of Total Loans	99.80%	99.80%
Performing Loans | Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 3,208,673	$ 3,106,421
% of Total Loans	99.90%	99.80%
Performing Loans | Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 2,620,360	$ 2,337,710
% of Total Loans	100.00%	99.80%
Performing Loans | Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,637,499	$ 1,423,891
% of Total Loans	100.00%	100.00%
Performing Loans | Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 191,162	$ 223,616
% of Total Loans	100.00%	100.00%
Performing Loans | Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 6,793,631	$ 6,539,633
% of Total Loans	99.70%	99.70%
Performing Loans | Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 408,128	$ 396,908
% of Total Loans	99.90%	99.90%
Performing Loans | Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 5,950,594	$ 5,709,529
% of Total Loans	99.70%	99.70%
Performing Loans | Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 147,007	$ 151,861
% of Total Loans	100.00%	99.90%
Performing Loans | Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 214,770	$ 205,800
% of Total Loans	99.90%	99.90%
Performing Loans | Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 73,132	$ 75,535
% of Total Loans	100.00%	100.00%
Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 29,180	$ 34,534
% of Total Loans	0.20%	0.20%
Non-Performing Loans | Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 11,420	$ 16,296
% of Total Loans	0.10%	0.20%
Non-Performing Loans | Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 5,879	$ 5,912
% of Total Loans	0.20%	0.20%
Non-Performing Loans | Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 4,635	$ 4,691
% of Total Loans	0.10%	0.20%
Non-Performing Loans | Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 906	$ 5,693
% of Total Loans	0.00%	0.20%
Non-Performing Loans | Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 0	$ 0
% of Total Loans	0.00%	0.00%
Non-Performing Loans | Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 0	$ 0
% of Total Loans	0.00%	0.00%
Non-Performing Loans | Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 17,760	$ 18,238
% of Total Loans	0.30%	0.40%
Non-Performing Loans | Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 435	$ 435
% of Total Loans	0.10%	0.10%
Non-Performing Loans | Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 17,084	$ 17,450
% of Total Loans	0.30%	0.30%
Non-Performing Loans | Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 71	$ 84
% of Total Loans	0.00%	0.10%
Non-Performing Loans | Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 134	$ 233
% of Total Loans	0.10%	0.10%
Non-Performing Loans | Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 36	$ 36
% of Total Loans	0.00%	0.00%