Mortgage Loans TDRs (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018 USD ($) contracts	Jun. 30, 2017 USD ($) contracts	Jun. 30, 2018 USD ($) contracts	Jun. 30, 2017 USD ($) contracts
Financing Receivable, Modifications [Line Items]
Average term extension of a single-family modified loan	128 months	155 months	133 months	154 months
Average interest rate reduction of a single-family modified loan	0.13%	0.67%	0.18%	0.80%
Number of loans troubled debt restructurings activity \| contracts	23,671	15,795	68,033	34,965
Recorded investment troubled debt restructurings activity \| $	$ 3,421	$ 2,206	$ 10,358	$ 4,852
Single-family [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	23,669	15,792	68,023	34,962
Recorded investment troubled debt restructurings activity \| $	$ 3,402	$ 2,189	$ 10,297	$ 4,835
Single-family [Member] \| Primary [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	21,820	14,148	63,499	31,383
Recorded investment troubled debt restructurings activity \| $	$ 3,148	$ 1,945	$ 9,672	$ 4,308
Single-family [Member] \| Government [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	26	45	74	106
Recorded investment troubled debt restructurings activity \| $	$ 2	$ 4	$ 6	$ 10
Single-family [Member] \| Alt-A [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	1,538	1,328	3,720	2,893
Recorded investment troubled debt restructurings activity \| $	$ 200	$ 194	$ 483	$ 418
Single-family [Member] \| Other [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	285	271	730	580
Recorded investment troubled debt restructurings activity \| $	$ 52	$ 46	$ 136	$ 99
Multifamily [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity \| contracts	2	3	10	3
Recorded investment troubled debt restructurings activity \| $	$ 19	$ 17	$ 61	$ 17

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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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