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Derivatives (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Derivative [Line Items] |  |  |  |  |
Derivative assets, fair value | $ 5,727 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue |  | $ 18,488 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue |  |
Derivative liabilities, fair value | (35,898)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue |  | (6,681)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue |  |
Notional amount of derivative instruments added | 7,950,000 dx_NotionalAmountofDerivativeInstrumentsAdded |  |  |  |
Notional amount of derivative instruments terminated | (215,000)dx_NotionalAmountofDerivativeInstrumentsTerminated |  |  |  |
Derivatives, notional amount | 17,525,000 invest_DerivativeNotionalAmount |  | 9,790,000 invest_DerivativeNotionalAmount |  |
Gain (loss) on derivative instruments, net | (53,393)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet |  | (10,076)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet |  | (908)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
Not Designated as Hedging Instrument [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative assets, aggregate notional amount | 440,000 us-gaap_DerivativeAssetNotionalAmount / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 575,000 us-gaap_DerivativeAssetNotionalAmount / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  |
Derivative liabilities, aggregate notional amount | 17,085,000 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 9,215,000 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  |
Interest Rate Swap [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative assets, fair value | 5,727 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  | 18,488 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |
Derivative liabilities, fair value | (3,002)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  | (1,336)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember |  |
Interest Rate Swap [Member] | Not Designated as Hedging Instrument [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative assets, aggregate notional amount | 440,000 us-gaap_DerivativeAssetNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 575,000 us-gaap_DerivativeAssetNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  |
Derivative liabilities, aggregate notional amount | 485,000 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 215,000 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  |
Eurodollar Future [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative assets, fair value | 0 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember |  |  |  |
Derivative liabilities, fair value | (32,896)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember |  | (5,345)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLiabilityAtFairValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember |  |
Notional amount of derivative instruments added | 7,600,000 dx_NotionalAmountofDerivativeInstrumentsAdded / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember |  |  |  |
Notional amount of derivative instruments terminated | 0 dx_NotionalAmountofDerivativeInstrumentsTerminated / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember |  |  |  |
Derivatives, notional amount | 16,600,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember |  | 9,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember |  |
Gain (loss) on derivative instruments, net | (27,551)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember |  | (19,391)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember |  | 0 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember
Eurodollar Future [Member] | Not Designated as Hedging Instrument [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivative assets, aggregate notional amount | 0 us-gaap_DerivativeAssetNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] |  |  |
Derivative liabilities, aggregate notional amount | 16,600,000 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | 9,000,000 us-gaap_DerivativeLiabilityNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EurodollarFutureMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] |
Long [Member] | Interest Rate Swap [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Notional amount of derivative instruments added | 275,000 dx_NotionalAmountofDerivativeInstrumentsAdded / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Notional amount of derivative instruments terminated | 0 dx_NotionalAmountofDerivativeInstrumentsTerminated / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Derivatives, notional amount | 275,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  | 0 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |
Gain (loss) on derivative instruments, net | 4,912 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  | 0 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  | 0 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 0 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext3748MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Pay Rate | 0.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext3748MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Receive Rate | 0.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext3748MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 250,000 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext4960MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Pay Rate | 2.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext4960MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Receive Rate | 2.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext4960MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 0 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext6172MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Pay Rate | 0.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext6172MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Receive Rate | 0.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext6172MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 73-84 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 0 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext7384MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Pay Rate | 0.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext7384MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Receive Rate | 0.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext7384MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 97-108 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 0 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext97108MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Pay Rate | 0.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext97108MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Receive Rate | 0.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext97108MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Long [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 109-120 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 25,000 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext109120MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Pay Rate | 3.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext109120MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Weighted-average Fixed Receive Rate | 3.00%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext109120MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_LongMember |  |  |  |
Short [Member] | Interest Rate Swap [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Notional amount of derivative instruments added | 75,000 dx_NotionalAmountofDerivativeInstrumentsAdded / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Notional amount of derivative instruments terminated | (215,000)dx_NotionalAmountofDerivativeInstrumentsTerminated / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Derivatives, notional amount | 650,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  | 790,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |
Gain (loss) on derivative instruments, net | (30,754)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  | 9,315 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  | (908)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 37-48 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 185,000 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext3748MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Pay Rate | 0.92%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext3748MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Receive Rate | 0.92%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext3748MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 49-60 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 235,000 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext4960MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Pay Rate | 1.45%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext4960MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Receive Rate | 1.45%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext4960MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 61-72 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 25,000 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext6172MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Pay Rate | 1.61%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext6172MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Receive Rate | 1.61%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext6172MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 73-84 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 25,000 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext7384MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Pay Rate | 2.19%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext7384MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Receive Rate | 2.19%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext7384MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 97-108 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | 30,000 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext97108MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Pay Rate | 1.93%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext97108MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Receive Rate | 1.93%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext97108MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Short [Member] | Interest Rate Swap [Member] | Interest Rate Swap Agreements, Maturities in Next 109-120 Months [Member] |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Derivatives, notional amount | $ 150,000 invest_DerivativeNotionalAmount / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext109120MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Pay Rate | 2.17%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext109120MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
Weighted-average Fixed Receive Rate | 2.17%us-gaap_DerivativeAverageFixedInterestRate / dx_DerivativeInstrumentMaturityScheduleAxis = dx_InterestRateSwapAgreementsMaturitiesinNext109120MonthsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  |
[1] | The Eurodollar futures aggregate notional amount represents the total notional of the 3-month contracts with expiration dates from 2015 to 2020. The maximum notional outstanding for any future 3-month period did not exceed $1,300,000 as of December 31, 2014 and $1,175,000 as of December 31, 2013.

X
- Definition Aggregate notional amount entered into in the period specified by the derivative(s). Expressed as an absolute value. + References No definition available. + Details
Name: | dx_NotionalAmountofDerivativeInstrumentsAdded
Namespace Prefix: | dx_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- Definition Aggregate notional amount terminated in the period specified by the derivative(s). Expressed as an absolute value. + References No definition available. + Details
Name: | dx_NotionalAmountofDerivativeInstrumentsTerminated
Namespace Prefix: | dx_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- 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
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