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Derivatives Volume of Activity (Details) $ in Thousands | 9 Months Ended
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Sep. 30, 2018 USD ($)
Not Designated as Hedging Instrument, Economic Hedge [Member] | TBA securities | Short [Member] |  |
Derivative [Line Items] |  |
Beginning of Period Notional Amount | $ 150,000 | [1]
Notional Amount of Derivative Instruments Added | 0 |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off | (150,000) |
End of Period Notional Amount | 0 | [1]
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps |  |
Derivative [Line Items] |  |
Beginning of Period Notional Amount | 5,675,000 | [2]
End of Period Notional Amount | 4,180,000 | [2]
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps | Long [Member] |  |
Derivative [Line Items] |  |
Beginning of Period Notional Amount | 100,000 |
Notional Amount of Derivative Instruments Added | 0 |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off | (100,000) |
End of Period Notional Amount | 0 |
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps | Short [Member] |  |
Derivative [Line Items] |  |
Beginning of Period Notional Amount | 5,775,000 |
Notional Amount of Derivative Instruments Added | 955,000 |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off | (2,550,000) |
End of Period Notional Amount | 4,180,000 |
Not Designated as Hedging Instrument, Economic Hedge [Member] | Eurodollar futures |  |
Derivative [Line Items] |  |
Beginning of Period Notional Amount | 1,950,000 | [3]
Notional Amount of Derivative Instruments Added | 0 | [3]
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off | (1,950,000) | [3]
End of Period Notional Amount | 0 | [3]
Not Designated as Hedging Instrument, Trading | TBA securities | Long [Member] |  |
Derivative [Line Items] |  |
Beginning of Period Notional Amount | 795,000 | [1]
Notional Amount of Derivative Instruments Added | 7,857,000 |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off | (7,891,000) |
End of Period Notional Amount | $ 761,000 | [1]
[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] | The net notional amounts included in the tables above represent pay-fixed interest rate swaps, net of any receive-fixed interest rate swaps, and include $1,525,000 and $2,655,000 of pay-fixed forward starting interest rate swaps as of September 30, 2018 and December 31, 2017, respectively.
[3] | The Eurodollar futures notional amounts represent the total notional of the 3-month contracts all of which expire in 2018. The maximum notional outstanding for any future 3-month period did not exceed $650,000 during the period indicated.

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