Mortgage Loans TDRs (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017 USD ($) contracts	Sep. 30, 2016 USD ($) contracts	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2016 USD ($) contracts
Financing Receivable, Modifications [Line Items]
Average term extension of a single-family modified loan	156 months	153 months	155 months	157 months
Average interest rate reduction of a single-family modified loan	0.35%	0.82%	0.66%	0.76%
Number of loans troubled debt restructurings activity \| contracts	14,853	15,934	49,818	52,319
Recorded investment troubled debt restructurings activity \| $	$ 2,166	$ 2,216	$ 7,018	$ 7,271
Single-family [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	14,848	15,932	49,810	52,313
Recorded investment troubled debt restructurings activity \| $	$ 2,084	$ 2,211	$ 6,919	$ 7,221
Single-family [Member] \| Primary [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	13,323	13,983	44,706	45,987
Recorded investment troubled debt restructurings activity \| $	$ 1,847	$ 1,922	$ 6,155	$ 6,282
Single-family [Member] \| Government [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	32	54	138	136
Recorded investment troubled debt restructurings activity \| $	$ 5	$ 5	$ 15	$ 14
Single-family [Member] \| Alt-A [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	1,229	1,578	4,122	5,112
Recorded investment troubled debt restructurings activity \| $	$ 182	$ 227	$ 600	$ 735
Single-family [Member] \| Other [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	264	317	844	1,078
Recorded investment troubled debt restructurings activity \| $	$ 50	$ 57	$ 149	$ 190
Multifamily [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	5	2	8	6
Recorded investment troubled debt restructurings activity \| $	$ 82	$ 5	$ 99	$ 50

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- Definition

The following represents the average term extension (disclosed in months) resulting from loan modifications.

+ References

No definition available.

+ Details

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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

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

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