Mortgage Loans TDRs (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($) contracts	Sep. 30, 2017 USD ($) contracts	Sep. 30, 2018 USD ($) contracts	Sep. 30, 2017 USD ($) contracts
Financing Receivable, Modifications [Line Items]
Average term extension of a single-family modified loan	98 months	156 months	104 months	155 months
Average interest rate reduction of a single-family modified loan	0.14%	0.35%	0.23%	0.66%
Number of loans troubled debt restructurings activity \| contracts	13,300	14,853	81,333	49,818
Recorded investment troubled debt restructurings activity \| $	$ 1,944	$ 2,166	$ 12,302	$ 7,018
Single-family [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	13,298	14,848	81,321	49,810
Recorded investment troubled debt restructurings activity \| $	$ 1,937	$ 2,084	$ 12,234	$ 6,919
Single-family [Member] \| Primary [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	12,291	13,323	75,790	44,706
Recorded investment troubled debt restructurings activity \| $	$ 1,797	$ 1,847	$ 11,469	$ 6,155
Single-family [Member] \| Government [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	21	32	95	138
Recorded investment troubled debt restructurings activity \| $	$ 3	$ 5	$ 9	$ 15
Single-family [Member] \| Alt-A [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	779	1,229	4,499	4,122
Recorded investment troubled debt restructurings activity \| $	$ 100	$ 182	$ 583	$ 600
Single-family [Member] \| Other [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	207	264	937	844
Recorded investment troubled debt restructurings activity \| $	$ 37	$ 50	$ 173	$ 149
Multifamily [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	2	5	12	8
Recorded investment troubled debt restructurings activity \| $	$ 7	$ 82	$ 68	$ 99

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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.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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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- Definition

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

+ References

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