Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 13,051,199	$ 12,959,272
Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	7,341,847	7,044,426
Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	1,609,800	1,538,240
Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	1,942,854	1,884,688
Commercial loans | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	2,243,465	2,115,513
Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	1,393,107	1,352,414
Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	152,621	153,571
Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	5,709,352	5,914,846
Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	299,351	295,953
Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	5,048,435	5,293,962
Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	109,845	101,394
Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	140,272	140,222
Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	111,449	83,315
Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 12,992,793	$ 12,930,216
% of Total Loans	99.60%	99.80%
Performing Loans | Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 7,308,619	$ 7,038,657
% of Total Loans	99.50%	99.90%
Performing Loans | Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,609,800	$ 1,538,240
% of Total Loans	100.00%	100.00%
Performing Loans | Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,911,457	$ 1,880,917
% of Total Loans	98.40%	99.80%
Performing Loans | Commercial loans | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 2,242,871	$ 2,115,184
% of Total Loans	100.00%	100.00%
Performing Loans | Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,391,870	$ 1,350,745
% of Total Loans	99.90%	99.90%
Performing Loans | Commercial loans | Land - acquisition & development
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 152,621	$ 153,571
% of Total Loans	100.00%	100.00%
Performing Loans | Consumer loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 5,684,174	$ 5,891,559
% of Total Loans	99.60%	99.60%
Performing Loans | Consumer loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 299,351	$ 295,953
% of Total Loans	100.00%	100.00%
Performing Loans | Consumer loans | Single-family residential
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 5,024,086	$ 5,271,531
% of Total Loans	99.50%	99.60%
Performing Loans | Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 109,402	$ 101,151
% of Total Loans	99.60%	99.80%
Performing Loans | Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 139,938	$ 139,669
% of Total Loans	99.80%	99.60%
Performing Loans | Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 111,397	$ 83,255
% of Total Loans	100.00%	99.90%
Non-Performing Loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 58,406	$ 29,056
% of Total Loans	0.40%	0.20%
Non-Performing Loans | Commercial loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 33,228	$ 5,769
% of Total Loans	0.50%	0.10%
Non-Performing Loans | Commercial loans | Multi-family
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 0	$ 0
% of Total Loans	0.00%	0.00%
Non-Performing Loans | Commercial loans | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 31,397	$ 3,771
% of Total Loans	1.60%	0.20%
Non-Performing Loans | Commercial loans | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 594	$ 329
% of Total Loans	0.00%	0.00%
Non-Performing Loans | Commercial loans | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 1,237	$ 1,669
% of Total Loans	0.10%	0.10%
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	$ 25,178	$ 23,287
% of Total Loans	0.40%	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	$ 24,349	$ 22,431
% of Total Loans	0.50%	0.40%
Non-Performing Loans | Consumer loans | Land - consumer lot loans
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 443	$ 243
% of Total Loans	0.40%	0.20%
Non-Performing Loans | Consumer loans | HELOC
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 334	$ 553
% of Total Loans	0.20%	0.40%
Non-Performing Loans | Consumer loans | Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Amount	$ 52	$ 60
% of Total Loans	0.00%	0.10%