Loans Receivable (excluding Covered Loans) - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	$ 8,422,772	$ 8,323,544
Current	8,299,296	8,203,280
30 days	28,093	19,762
60 days	8,990	27,907
90 days	86,393	72,595
Total	123,476wfsl_LoansAndLeasesReceivablePastDue	120,264wfsl_LoansAndLeasesReceivablePastDue
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.47%	1.44%
Non-Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	8,167,625	8,063,364
Current	8,052,077	7,947,551
30 days	24,584	19,477
60 days	8,463	26,449
90 days	82,501	69,887
Total	115,548	115,813
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.41%	1.44%
Non-Acquired Loans [Member] \| Single-Family Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	5,605,917	5,557,753
Current	5,518,834	5,467,239
30 days	17,503	15,926
60 days	7,804	9,139
90 days	61,776	65,449
Total	87,083	90,514
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.55%	1.63%
Non-Acquired Loans [Member] \| Construction - Speculative [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	95,367	87,035
Current	95,199	87,035
30 days	168	0
60 days	0	0
90 days	0	0
Total	168	0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.18%	0.00%
Non-Acquired Loans [Member] \| Construction - Custom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	191,787	192,098
Current	191,670	191,262
30 days	114	836
60 days	3	0
90 days	0	0
Total	117	836
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.06%	0.44%
Non-Acquired Loans [Member] \| Land - Acquisition & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	70,347	68,066
Current	68,008	67,911
30 days	2,339	155
60 days	0	0
90 days	0	0
Total	2,339	155
Ratio of past due loans to total loans, net of charge-offs and LIPs	3.32%	0.23%
Non-Acquired Loans [Member] \| Land - Consumer Lot Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	104,444	108,589
Current	100,639	104,571
30 days	596	1,246
60 days	368	304
90 days	2,841	2,468
Total	3,805	4,018
Ratio of past due loans to total loans, net of charge-offs and LIPs	3.64%	3.70%
Non-Acquired Loans [Member] \| Multi-Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	906,295	892,196
Current	905,532	891,372
30 days	0	205
60 days	0	16
90 days	763	603
Total	763	824
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.08%	0.09%
Non-Acquired Loans [Member] \| Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	559,808	529,453
Current	541,939	513,409
30 days	2,607	67
60 days	0	15,118
90 days	15,262	859
Total	17,869	16,044
Ratio of past due loans to total loans, net of charge-offs and LIPs	3.19%	3.03%
Non-Acquired Loans [Member] \| Commercial & Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	388,588	379,226
Current	387,149	377,848
30 days	339	53
60 days	0	1,318
90 days	1,100	7
Total	1,439	1,378
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.37%	0.36%
Non-Acquired Loans [Member] \| HELOC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	118,143	116,262
Current	117,362	115,262
30 days	162	335
60 days	58	292
90 days	561	373
Total	781	1,000
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.66%	0.86%
Non-Acquired Loans [Member] \| Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	126,929	132,686
Current	125,745	131,642
30 days	756	654
60 days	230	262
90 days	198	128
Total	1,184	1,044
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.93%	0.79%
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	180,501	180,892
Current	176,954	178,945
30 days	1,445	126
60 days	97	474
90 days	2,005	1,347
Total	3,547	1,947
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.97%	1.08%
Acquired Loans [Member] \| Single-Family Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	11,164	11,716
Current	10,907	11,693
30 days	232	0
60 days	0	0
90 days	25	23
Total	257	23
Ratio of past due loans to total loans, net of charge-offs and LIPs	2.30%	0.20%
Acquired Loans [Member] \| Land - Acquisition & Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	872	905
Current	872	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 Loans [Member] \| Land - Consumer Lot Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	2,495	2,502
Current	1,686	2,132
30 days	560	0
60 days	0	370
90 days	249	0
Total	809	370
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%	14.79%
Acquired Loans [Member] \| Multi-Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	2,954	2,999
Current	2,954	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 Loans [Member] \| Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	92,066	97,715
Current	91,211	96,948
30 days	0	104
60 days	97	0
90 days	758	663
Total	855	767
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.93%	0.78%
Acquired Loans [Member] \| Commercial & Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	58,832	51,329
Current	58,733	51,229
30 days	0	0
60 days	0	100
90 days	99	0
Total	99	100
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.17%	0.19%
Acquired Loans [Member] \| HELOC [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	7,749	8,056
Current	7,290	8,056
30 days	241	0
60 days	0	0
90 days	218	0
Total	459	0
Ratio of past due loans to total loans, net of charge-offs and LIPs	5.92%	0.00%
Acquired Loans [Member] \| Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	4,369	5,670
Current	3,301	4,983
30 days	412	22
60 days	0	4
90 days	656	661
Total	1,068	687
Ratio of past due loans to total loans, net of charge-offs and LIPs	24.44%	12.12%
Credit Impaired Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	74,646	79,288
Current	70,265	76,784
30 days	2,064	159
60 days	430	984
90 days	1,887	1,361
Total	4,381	2,504
Ratio of past due loans to total loans, net of charge-offs and LIPs	5.87%	3.16%
Credit Impaired Acquired Loans [Member] \| Single-Family Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	323	325
Current	323	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 loan, net of charge-offs and LIP	1,533	1,581
Current	1,533	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 loan, net of charge-offs and LIP	60,280	63,713
Current	56,301	61,713
30 days	2,064	152
60 days	430	909
90 days	1,485	939
Total	3,979	2,000
Ratio of past due loans to total loans, net of charge-offs and LIPs	6.60%	3.14%
Credit Impaired Acquired Loans [Member] \| Commercial & Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	3,255	3,477
Current	3,255	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 loan, net of charge-offs and LIP	9,201	10,138
Current	8,799	9,641
30 days	0	0
60 days	0	75
90 days	402	422
Total	402	497
Ratio of past due loans to total loans, net of charge-offs and LIPs	4.37%	4.90%
Credit Impaired Acquired Loans [Member] \| Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loan, net of charge-offs and LIP	54	54
Current	54	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