Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 12,898,775	$ 13,312,388
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	6,836,049	6,564,698
Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	1,509,837	1,422,674
Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	1,700,568	1,631,170
Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	2,138,462	1,268,695
Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	1,334,948	2,038,052
Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	152,234	204,107
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	6,062,726	6,747,690
Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	278,182	540,741
Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	5,452,424	5,835,194
Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	99,255	99,694
Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	141,165	142,178
Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	91,700	129,883
Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 12,863,774	$ 13,278,657
% of Total Loans	99.70%	99.70%
Performing Loans | Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 6,827,131
% of Total Loans	99.90%
Performing Loans | Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,509,613	$ 1,422,674
% of Total Loans	100.00%	100.00%
Performing Loans | Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,697,152	$ 1,625,335
% of Total Loans	99.80%	99.60%
Performing Loans | Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 2,136,615	$ 1,267,403
% of Total Loans	99.90%	99.90%
Performing Loans | Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,331,595	$ 2,038,052
% of Total Loans	99.70%	100.00%
Performing Loans | Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 152,156	$ 203,938
% of Total Loans	99.90%	99.90%
Performing Loans | Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 6,036,643
% of Total Loans	99.60%
Performing Loans | Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 278,182	$ 540,741
% of Total Loans	100.00%	100.00%
Performing Loans | Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 5,427,548	$ 5,809,923
% of Total Loans	99.50%	99.60%
Performing Loans | Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 98,978	$ 99,448
% of Total Loans	99.70%	99.80%
Performing Loans | Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 140,299	$ 141,271
% of Total Loans	99.40%	99.40%
Performing Loans | Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 91,636	$ 129,872
% of Total Loans	99.90%	100.00%
Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 35,001	$ 33,731
% of Total Loans	0.30%	0.30%
Non-Performing Loans | Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 8,918
% of Total Loans	0.10%
Non-Performing Loans | Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 224	$ 0
% of Total Loans	0.00%	0.00%
Non-Performing Loans | Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 3,416	$ 5,835
% of Total Loans	0.20%	0.40%
Non-Performing Loans | Commercial loans | Commercial & industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,847	$ 1,292
% of Total Loans	0.10%	0.10%
Non-Performing Loans | Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 3,353	$ 0
% of Total Loans	0.30%	0.00%
Non-Performing Loans | Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 78	$ 169
% of Total Loans	0.10%	0.10%
Non-Performing Loans | Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 26,083
% of Total Loans	0.40%
Non-Performing Loans | Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 0	$ 0
% of Total Loans	0.00%	0.00%
Non-Performing Loans | Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 24,876	$ 25,271
% of Total Loans	0.50%	0.40%
Non-Performing Loans | Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 277	$ 246
% of Total Loans	0.30%	0.20%
Non-Performing Loans | Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 866	$ 907
% of Total Loans	0.60%	0.60%
Non-Performing Loans | Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 64	$ 11
% of Total Loans	0.10%	0.00%