v2.4.1.9

Capital (Dividends) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		1 Months Ended	2 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Apr. 30, 2015	May 31, 2015
Dividends [Line Items]
Cash Dividends per Share	$ 0.025
Total cash dividend and interest expense paid	$ 598	$ 672
Cash dividends on capital stock	215us-gaap_DividendsCommonStockCash	231us-gaap_DividendsCommonStockCash
Cash dividends on MRCS	383	441
Regulatory Restrictions on issuance of capital stock dividends, Minimum excess capital stock to assets, Percent	1.00%
Excess Capital	1,605,476us-gaap_ExcessCapital
Excess Capital to Assets	4.80%fhlbsea_ExcessCapitalToAssets
Subsequent Event [Member]
Dividends [Line Items]
Cash Dividends per Share			$ 0.025	$ 0.017
Total cash dividend and interest expense paid			576
Cash dividends on capital stock			215
Cash dividends on MRCS			$ 361

Cash Dividend and Interest Expense Paid on Capital Stock and Mandatory Redeemable Capital Stock

+ References

No definition available.

+ Details

Name:	fhlbsea_CashDividendandInterestExpensePaid
Namespace Prefix:	fhlbsea_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	fhlbsea_DividendsLineItems
Namespace Prefix:	fhlbsea_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Excess Capital to Assets

+ References

No definition available.

+ Details

Name:	fhlbsea_ExcessCapitalToAssets
Namespace Prefix:	fhlbsea_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Regulatory Restrictions on Payment of Capital Stock Dividends, Excess Stock to Assets, Percent

+ References

No definition available.

+ Details

Name:	fhlbsea_RegulatoryRestrictionsonissuanceofcapitalstockdividendsMinimumexcesscapitalstocktoassetsPercent
Namespace Prefix:	fhlbsea_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate dividends paid during the period for each share of common stock outstanding.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

Name:	us-gaap_CommonStockDividendsPerShareCashPaid
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.

+ References

+ Details

Name:	us-gaap_DividendsCommonStockCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount by which the company's Total Capital exceeds the minimum required for capital adequacy purposes.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

+ Details

Name:	us-gaap_ExcessCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Interest expense incurred during the reporting period on capital securities issued by the reporting entity.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 8

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.8)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

Name:	us-gaap_InterestExpenseCapitalSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Federal Home Loan Bank of Seattle 10-Q