Mortgage Loans Loans in Mortgage Portfolio (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	$ 3,176,703		$ 3,155,703
Cost basis and fair value adjustments, net	39,817		41,906
Allowance for loan losses for loans held for investment	(18,734)	$ (22,129)	(19,084)	$ (23,465)
Total mortgage loans	3,197,786		3,178,525
Carrying value of loans redesignated from HFI to HFS	7,400	2,500
Carrying value of loans redesignated from HFS to HFI	18	35
Sale of loans, unpaid principal balance	748	$ 93
Single-family [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	2,907,387		2,890,634
Recorded investment of single-family mortgage loans referred to foreclosure	12,700		13,000
Multifamily [Member]
Loans in Mortgage Portfolio [Line Items]
Unpaid principal balance of mortgage loans	$ 269,316		$ 265,069

The amount of financing receivable to held-for-investment during the reporting period.

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Represents premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield and fair value adjustments.

+ References

No definition available.

+ Details

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- Definition

Represents principal amount outstanding of mortgage loans.

+ References

No definition available.

+ Details

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- Definition

The reserve to cover probable credit losses related to specifically identified fixed maturity real estate loans as well as probable credit losses inherent in the remainder of that loan portfolio.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82921503&loc=d3e21338-158488

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- Definition

The amount of financing receivables reclassified to held for sale during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

The amount of any significant sales of financing receivables during the reporting period.

+ References

+ Details

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- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL49126937-111524

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