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Derivatives TBA securities (Details) - TBA securities - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Short [Member] | Not Designated as Hedging Instrument, Economic Hedge [Member] |  |  |
Derivative [Line Items] |  |  |
Derivative, Notional Amount | [1] | $ 0 | $ 150,000
Cost Basis,TBA | [2] | 0 | (153,797)
Market value, TBA | [3] | 0 | (154,066)
Net carrying value, TBA | [4] | 0 | (269)
Long [Member] | Not Designated as Hedging Instrument, Trading |  |  |
Derivative [Line Items] |  |  |
Derivative, Notional Amount | [1] | 761,000 | 795,000
Cost Basis,TBA | [2] | 780,865 | 829,425
Market value, TBA | [3] | 779,697 | 830,908
Net carrying value, TBA | [4] | $ (1,168) | $ 1,483
[1] | Notional amount represents the par value (or principal balance) of the underlying Agency MBS as if settled as of the end of the period.
[2] | Implied cost basis represents the forward price to be paid for the underlying Agency MBS as if settled as of end of the period.
[3] | Implied market value represents the estimated fair value of the underlying Agency MBS as if settled as of the end of the period.
[4] | Net carrying value is the amount included on the consolidated balance sheets within “derivative assets (liabilities)” and represents the difference between the implied market value and the implied cost basis of the TBA security as of the end of the period.

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- Definition Represents the forward price to be paid for the underlying security as if settled. + References No definition available. + Details
Name: | dx_CostBasisTBA
Namespace Prefix: | dx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Represents the current estimated fair value of the security underlying the forward contract. + References No definition available. + Details
Name: | dx_MarketvalueTBA
Namespace Prefix: | dx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Represents the difference between the market value and the cost basis of the TBA contract and is the net amount at which the TBA position is recorded on balance sheet as derivative asset (liability). + References No definition available. + Details
Name: | dx_NetcarryingvalueTBA
Namespace Prefix: | dx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
Name: | invest_DerivativeNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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

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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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- Details
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Data Type: | na
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