Repurchase Agreements (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 2,594,683	$ 4,752,348
Weighted Average Rate	0.36%	2.01%
Fair Value of Collateral Pledged	$ 2,766,132	$ 5,024,625
Payable for unsettled securities	190	6,180
Agency \| Commercial Mortgage Backed Securities
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 275,473	$ 1,735,848
Weighted Average Rate	0.23%	1.98%
Fair Value of Collateral Pledged	$ 296,572	$ 1,901,452
Agency \| Residential Mortgage Backed Securities
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 1,975,736	$ 2,594,645
Weighted Average Rate	0.24%	1.96%
Fair Value of Collateral Pledged	$ 2,069,035	$ 2,647,638
Agency \| Interest-Only-Strip
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 221,013	$ 255,912
Weighted Average Rate	1.04%	2.30%
Fair Value of Collateral Pledged	$ 254,886	$ 282,522
Non-Agency \| Interest-Only-Strip
Assets and Associated Liabilities of Transfers Accounted for as Secured Borrowings [Line Items]
Assets Sold under Agreements to Repurchase, Repurchase Liability	$ 122,461	$ 165,943
Weighted Average Rate	1.26%	2.67%
Fair Value of Collateral Pledged	$ 145,639	$ 193,013

Amount payable to other broker-dealer and clearing organization, including, but not limited to, security failed-to-receive, deposit received for security loaned, open transaction, and floor-brokerage payable.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

+ Details

Name:	srt_PayablesToBrokerDealersAndClearingOrganizations
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Name:	us-gaap_AssetsAndAssociatedLiabilitiesOfTransfersAccountedForAsSecuredBorrowingsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount of assets sold under agreements to repurchase when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under repurchase agreements exceeds 10 percent of total assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseCarryingAmounts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseRepurchaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	dx_TypeofIssuerAxis=dx_AgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestOnlyStripMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dx_TypeofIssuerAxis=dx_NonAgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: