Credit Enhancement Receivable, Credit Enhancement Deposit and Contingent Liability (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit
Credit risk loans	$ 22,658,120,000		$ 22,658,120,000		$ 20,209,577,000
Credit enhancement receivable	0		0		0
Deposit in Fannie Mae restricted liquidity account	25,000,000.0		25,000,000.0		25,000,000.0
Credit Risk
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit
Maximum DB Cayman credit protection	28,762,000		28,762,000		29,253,000
DB Cayman \| Credit Risk
Credit Enhancement Receivable Contingent Liability And Credit Enhancement Deposit
Reimbursements under serving agreement	0	$ 0	0	$ 0
Credit risk loans	$ 6,698,151,000		6,698,151,000		5,835,163,000
Maximum pre-credit enhancement loss exposure			9,587,000		9,751,000
Maximum loss exposure without any form of credit protection			$ 6,688,564,000		5,825,412,000
Percentage of contingent payment (in percentage)	50.00%		50.00%
Contingent liability	$ 12,200,000		$ 12,200,000		$ 11,800,000

Concentration risk credit risk financial instrument maximum exposure without credit protection.

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- Definition

Credit enhancement receivable.

+ References

No definition available.

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- Definition

Credit enhancement receivable contingent liability and credit enhancement deposit.

+ References

No definition available.

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- Definition

Percentage of contingent payment.

+ References

No definition available.

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- Definition

Reimbursements under serving agreement.

+ References

No definition available.

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- Definition

Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13537-108611

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- Definition

This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

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- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12069-110248

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- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14326-108349

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- Definition

The amount of an asset, typically cash, provided to a counterparty to provide certain assurance of performance by the entity pursuant to the terms of a written or oral agreement, such as a lease.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8,17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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