Fair Value Measurements - Investments by Contractual Maturity Date (Details) (USD $)	3 Months Ended	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Sep. 30, 2014
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 2,710,275,000		$ 2,710,275,000	$ 3,016,432,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	50,132,000		50,132,000	37,273,000
Gross unrealized loss	(3,501,000)		(3,501,000)	(4,263,000)
Fair Value, Available-for-sale securities
Fair Value	2,756,906,000us-gaap_AvailableForSaleSecurities		2,756,906,000us-gaap_AvailableForSaleSecurities	3,049,442,000us-gaap_AvailableForSaleSecurities
Yield, Available-for-sale securities
Yield	1.99%wfsl_AvailableForSaleSecuritiesYield		1.99%wfsl_AvailableForSaleSecuritiesYield	1.99%wfsl_AvailableForSaleSecuritiesYield
Yield, Held-to-maturity securities
Investments	4,190,056,000us-gaap_Investments		4,190,056,000us-gaap_Investments	4,564,697,000us-gaap_Investments
Gross unrealized gains on investments	61,864,000wfsl_GrossUnrealizedGainsOnInvestments		61,864,000wfsl_GrossUnrealizedGainsOnInvestments	42,128,000wfsl_GrossUnrealizedGainsOnInvestments
Gross unrealized losses on investments	(14,537,000)wfsl_GrossUnrealizedLossesOnInvestments		(14,537,000)wfsl_GrossUnrealizedLossesOnInvestments	(58,165,000)wfsl_GrossUnrealizedLossesOnInvestments
Total financial assets	4,237,383,000us-gaap_InvestmentsFairValueDisclosure		4,237,383,000us-gaap_InvestmentsFairValueDisclosure	4,548,660,000us-gaap_InvestmentsFairValueDisclosure
Yield on investments	2.39%wfsl_Yield		2.39%wfsl_Yield	2.38%wfsl_Yield
Available-for-sale securities sold	0	0
Term of Contractual Due Dates of Substantially All Mortgage-Backed Securities			10 years
US Treasury and Government [Member]
Amortized Cost, Available-for-sale securities
1 to 5 years	142,283,000		142,283,000	171,154,000
5 to 10 years	107,717,000		107,717,000	203,317,000
Over 10 years	281,951,000		281,951,000	354,828,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	2,300,000		2,300,000	2,585,000
Gross unrealized gains, 5 to 10 years	403,000		403,000	300,000
Gross unrealized gains, Over 10 years	436,000		436,000	1,028,000
Gross unrealized losses, 1 to 5 years	(119,000)		(119,000)	(748,000)
Gross unrealized losses, 5 to 10 years	0		0	(102,000)
Gross unrealized losses, Over 10 years	(526,000)		(526,000)	(419,000)
Fair Value, Available-for-sale securities
1 to 5 years	144,464,000		144,464,000	172,991,000
5 to 10 years	108,120,000		108,120,000	203,515,000
Over 10 years	281,861,000		281,861,000	355,437,000
Yield, Available-for-sale securities
1 to 5 years	1.41%		1.41%	1.26%
5 to 10 years	1.19%		1.19%	1.45%
Over 10 years	1.14%		1.14%	1.25%
Equity Securities [Member]
Amortized Cost, Available-for-sale securities
Within 1 year	500,000		500,000
1 to 5 years	100,000,000		100,000,000	100,500,000
5 to 10 years	0		0
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	17,000		17,000
Gross unrealized gains, 1 to 5 years	1,760,000		1,760,000	887,000
Gross unrealized gains, 5 to 10 years	0		0
Gross unrealized losses, Within 1 year	0		0
Gross unrealized losses, 1 to 5 years	0		0	0
Gross unrealized losses, 5 to 10 years	0		0
Fair Value, Available-for-sale securities
Within 1 year	517,000		517,000
1 to 5 years	101,760,000		101,760,000	101,387,000
5 to 10 years	0		0
Yield, Available-for-sale securities
Within 1 year	1.80%		1.80%
1 to 5 years	1.90%		1.90%	1.90%
5 to 10 years	0.00%		0.00%
Corporate Debt Securities [Member]
Amortized Cost, Available-for-sale securities
Within 1 year	15,000,000		15,000,000	15,000,000
1 to 5 years	327,715,000		327,715,000	302,540,000
5 to 10 years	133,271,000		133,271,000	138,201,000
Over 10 years	50,000,000		50,000,000	50,000,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	0		0	75,000
Gross unrealized gains, 1 to 5 years	1,860,000		1,860,000	2,372,000
Gross unrealized gains, 5 to 10 years	1,432,000		1,432,000	1,789,000
Gross unrealized gains, Over 10 years	250,000		250,000	0
Gross unrealized losses, Within 1 year	(15,000)		(15,000)	0
Gross unrealized losses, 1 to 5 years	0		0	0
Gross unrealized losses, 5 to 10 years	(949,000)		(949,000)	(970,000)
Gross unrealized losses, Over 10 years	0		0	0
Fair Value, Available-for-sale securities
Within 1 year	14,985,000		14,985,000	15,075,000
1 to 5 years	329,575,000		329,575,000	304,912,000
5 to 10 years	133,754,000		133,754,000	139,020,000
Over 10 years	50,250,000		50,250,000	50,000,000
Yield, Available-for-sale securities
Within 1 year	1.00%		1.00%	1.00%
1 to 5 years	0.74%		0.74%	0.71%
5 to 10 years	1.48%		1.48%	1.43%
Over 10 years	3.00%		3.00%	3.00%
Municipal Bonds [Member]
Amortized Cost, Available-for-sale securities
Over 10 years	20,392,000		20,392,000	20,402,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Over 10 years	3,621,000		3,621,000	3,279,000
Gross unrealized losses, Over 10 years	0		0	0
Fair Value, Available-for-sale securities
Over 10 years	24,013,000		24,013,000	23,681,000
Yield, Available-for-sale securities
Over 10 years	6.45%		6.45%	6.45%
Agency Pass-through Certificates [Member]
Amortized Cost, Available-for-sale securities
Without single maturity date	1,420,784,000		1,420,784,000	1,561,639,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	38,053,000		38,053,000	24,893,000
Gross unrealized losses, Without single maturity date	(1,892,000)		(1,892,000)	(2,024,000)
Fair Value, Available-for-sale securities
Without single maturity date	1,456,945,000		1,456,945,000	1,584,508,000
Yield, Available-for-sale securities
Without single maturity date	2.57%		2.57%	2.57%
Amortized Cost, Held-to-maturity securities
Without single maturity date	1,479,781,000		1,479,781,000	1,548,265,000
Gross Unrealized Gains / Losses, Held-to-maturity securities
Gross unrealized gains, Without single maturity date	11,732,000		11,732,000	4,855,000
Gross unrealized losses, Without single maturity date	(11,036,000)		(11,036,000)	(53,902,000)
Fair Value, Held-to-maturity securities
Without single maturity date	1,480,477,000		1,480,477,000	1,499,218,000
Yield, Held-to-maturity securities
Without a single maturity date	3.13%		3.13%	3.13%
Commercial Mortgage Backed Securities [Member]
Amortized Cost, Available-for-sale securities
Without single maturity date	110,662,000		110,662,000	98,851,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	0		0	65,000
Gross unrealized losses, Without single maturity date	0		0	0
Fair Value, Available-for-sale securities
Without single maturity date	$ 110,662,000		$ 110,662,000	$ 98,916,000
Yield, Available-for-sale securities
Without single maturity date	1.45%		1.45%	1.49%

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562