Loans Receivable (excluding Covered Loans) - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 8,587,919	$ 8,323,544
Current	8,490,210	8,203,280
30 days	25,861	19,762
60 days	11,778	27,907
90 days	60,070	72,595
Total	97,709wfsl_LoansAndLeasesReceivablePastDue	120,264wfsl_LoansAndLeasesReceivablePastDue
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.14%	1.44%
Non-Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	8,348,895	8,063,364
Current	8,257,291	7,947,551
30 days	24,302	19,477
60 days	9,701	26,449
90 days	57,601	69,887
Total	91,604	115,813
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.10%	1.44%
Non-Acquired Loans [Member] \| Single-Family Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	5,532,553	5,557,753
Current	5,450,820	5,467,239
30 days	19,822	15,926
60 days	8,082	9,139
90 days	53,829	65,449
Total	81,733	90,514
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.48%	1.63%
Non-Acquired Loans [Member] \| Construction - Speculative [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	105,392	87,035
Current	103,044	87,035
30 days	2,348	0
60 days	0	0
90 days	0	0
Total	2,348	0
Ratio of past due loans to total loans, net of charge-offs and LIPs	2.23%	0.00%
Non-Acquired Loans [Member] \| Construction - Custom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	201,777	192,098
Current	200,608	191,262
30 days	355	836
60 days	814	0
90 days	0	0
Total	1,169	836
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.58%	0.44%
Non-Acquired Loans [Member] \| Land - Acquisition & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	90,838	68,066
Current	90,838	67,911
30 days	0	155
60 days	0	0
90 days	0	0
Total	0	155
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	0.23%
Non-Acquired Loans [Member] \| Land - Consumer Lot Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	102,003	108,589
Current	98,717	104,571
30 days	466	1,246
60 days	478	304
90 days	2,342	2,468
Total	3,286	4,018
Ratio of past due loans to total loans, net of charge-offs and LIPs	3.22%	3.70%
Non-Acquired Loans [Member] \| Multi-Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	941,122	892,196
Current	940,175	891,372
30 days	620	205
60 days	0	16
90 days	327	603
Total	947	824
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.10%	0.09%
Non-Acquired Loans [Member] \| Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	627,843	529,453
Current	627,209	513,409
30 days	62	67
60 days	93	15,118
90 days	479	859
Total	634	16,044
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.10%	3.03%
Non-Acquired Loans [Member] \| Commercial & Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	407,763	379,226
Current	407,698	377,848
30 days	65	53
60 days	0	1,318
90 days	0	7
Total	65	1,378
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.02%	0.36%
Non-Acquired Loans [Member] \| HELOC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	120,904	116,262
Current	120,098	115,262
30 days	150	335
60 days	74	292
90 days	582	373
Total	806	1,000
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.67%	0.86%
Non-Acquired Loans [Member] \| Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	218,700	132,686
Current	218,084	131,642
30 days	414	654
60 days	160	262
90 days	42	128
Total	616	1,044
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.28%	0.79%
Acquired Non-Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	169,524	180,892
Current	165,858	178,945
30 days	1,172	126
60 days	1,561	474
90 days	933	1,347
Total	3,666	1,947
Ratio of past due loans to total loans, net of charge-offs and LIPs	2.16%	1.08%
Acquired Non-Impaired Loans [Member] \| Single-Family Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	10,976	11,716
Current	10,819	11,693
30 days	133	0
60 days	0	0
90 days	24	23
Total	157	23
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.43%	0.20%
Acquired Non-Impaired Loans [Member] \| Land - Acquisition & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	728	905
Current	728	905
30 days	0	0
60 days	0	0
90 days	0	0
Total	0	0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	0.00%
Acquired Non-Impaired Loans [Member] \| Land - Consumer Lot Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	2,474	2,502
Current	2,139	2,132
30 days	218	0
60 days	0	370
90 days	117	0
Total	335	370
Ratio of past due loans to total loans, net of charge-offs and LIPs	13.54%	14.79%
Acquired Non-Impaired Loans [Member] \| Multi-Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	2,912	2,999
Current	2,912	2,999
30 days	0	0
60 days	0	0
90 days	0	0
Total	0	0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	0.00%
Acquired Non-Impaired Loans [Member] \| Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	87,290	97,715
Current	85,063	96,948
30 days	492	104
60 days	1,561	0
90 days	174	663
Total	2,227	767
Ratio of past due loans to total loans, net of charge-offs and LIPs	2.55%	0.78%
Acquired Non-Impaired Loans [Member] \| Commercial & Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	55,670	51,329
Current	55,670	51,229
30 days	0	0
60 days	0	100
90 days	0	0
Total	0	100
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	0.19%
Acquired Non-Impaired Loans [Member] \| HELOC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	6,700	8,056
Current	6,201	8,056
30 days	275	0
60 days	0	0
90 days	224	0
Total	499	0
Ratio of past due loans to total loans, net of charge-offs and LIPs	7.45%	0.00%
Acquired Non-Impaired Loans [Member] \| Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	2,774	5,670
Current	2,326	4,983
30 days	54	22
60 days	0	4
90 days	394	661
Total	448	687
Ratio of past due loans to total loans, net of charge-offs and LIPs	16.15%	12.12%
Credit Impaired Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	69,500	79,288
Current	67,061	76,784
30 days	387	159
60 days	516	984
90 days	1,536	1,361
Total	2,439	2,504
Ratio of past due loans to total loans, net of charge-offs and LIPs	3.51%	3.16%
Credit Impaired Acquired Loans [Member] \| Single-Family Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	322	325
Current	322	325
30 days	0	0
60 days	0	0
90 days	0	0
Total	0	0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	0.00%
Credit Impaired Acquired Loans [Member] \| Land - Acquisition & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	1,395	1,581
Current	1,395	1,581
30 days	0	0
60 days	0	0
90 days	0	0
Total	0	0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	0.00%
Credit Impaired Acquired Loans [Member] \| Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	56,717	63,713
Current	54,366	61,713
30 days	387	152
60 days	516	909
90 days	1,448	939
Total	2,351	2,000
Ratio of past due loans to total loans, net of charge-offs and LIPs	4.15%	3.14%
Credit Impaired Acquired Loans [Member] \| Commercial & Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	2,179	3,477
Current	2,179	3,470
30 days	0	7
60 days	0	0
90 days	0	0
Total	0	7
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	0.20%
Credit Impaired Acquired Loans [Member] \| HELOC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	8,836	10,138
Current	8,748	9,641
30 days	0	0
60 days	0	75
90 days	88	422
Total	88	497
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.00%	4.90%
Credit Impaired Acquired Loans [Member] \| Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	51	54
Current	51	54
30 days	0	0
60 days	0	0
90 days	0	0
Total	$ 0	$ 0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	0.00%

Loans Receivable (excluding Covered Loans) - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) (USD $)
In Thousands, unless otherwise specified