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(Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative assets, fair value |  | $ 6,563 |  |  |  | $ 2,940 |  |  |  |  | $ 6,563 | $ 2,940 |  |
Derivative liabilities, fair value |  | (1,218) |  |  |  | (269) |  |  |  |  | (1,218) | (269) |  |
Gain (loss) on derivative instruments, net |  | $ (81,981) | $ 19,499 | $ 20,667 | $ 38,354 | $ 12,678 |  | $ 5,993 | $ (15,802) | $ 175 | $ (3,461) | $ 3,044 |  | $ (5,606)
Derivative, Average Fixed Interest Rate, Current Effective |  | 2.29% |  |  |  | 1.36% |  |  |  |  | 2.29% | 1.36% |  |
Forward starting interest rate swap |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  | $ (775,000) |  |  |  | $ (2,655,000) |  |  |  |  | $ (775,000) | $ (2,655,000) |  |
Eurodollar futures |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  |  |  |  |  |  |  |  |  |  | 1,887 | 821 |  |
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  |  |  |  |  |  |  |  |  |  |  |  | (4,815)
U.S. Treasury futures |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  |  |  |  |  |  |  |  |  | (4,609) |  |  |
Options on U.S. Treasury futures |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  |  |  |  |  |  |  |  |  | (658) |  |  |
Long [Member] | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  |  |  |  |  |  |  |  |  |  |  | 23 |  | 2,515
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  |  |  |  |  |  |  |  |  | (1,658) |  |  |
Long [Member] | TBA securities |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  |  |  |  |  |  |  |  |  |  |  | 5,757 |  | 0
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  |  |  |  |  |  |  |  |  | (10,737) |  |  |
Short [Member] | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  |  |  |  |  |  |  |  |  |  | 12,021 |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  |  |  |  |  |  |  |  |  |  | (2,655) |  | (3,306)
Short [Member] | TBA securities |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  |  |  |  |  |  |  |  |  |  | 293 |  |  | $ 0
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  |  |  |  |  |  |  |  |  |  | (902) |  |
Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative assets, fair value |  | 324 |  |  |  | 791 |  |  |  |  | 324 | 791 |  |
Derivative, Notional Amount | [1] | $ (4,515,000) |  |  |  | $ (5,675,000) |  |  |  |  | $ (4,515,000) | $ (5,675,000) |  |
Derivative, Average Fixed Interest Rate | [2] | 2.35% |  |  |  | 1.71% |  |  |  |  | 2.35% | 1.71% |  |
Derivative, Average Remaining Maturity |  |  |  |  |  |  |  |  |  |  | 5 years 6 months | 3 years 1 month 6 days |  |
Derivative, Fair Value, Net | [3] | $ 324 |  |  |  | $ 791 |  |  |  |  | $ 324 | $ 791 |  |
Not Designated as Hedging Instrument, Economic Hedge [Member] | Eurodollar futures |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative assets, fair value |  | 0 |  |  |  | 666 |  |  |  |  | 0 | 666 |  |
Derivative, Notional Amount | [4] | 0 |  |  |  | (1,950,000) |  |  |  |  | 0 | (1,950,000) |  |
Notional Amount of Derivative Instruments Added | [4] |  |  |  |  |  |  |  |  |  | 0 |  |  |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off | [4] |  |  |  |  |  |  |  |  |  | (1,950,000) |  |  |
Not Designated as Hedging Instrument, Economic Hedge [Member] | U.S. Treasury futures |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  | (50,000) |  |  |  | 0 |  |  |  |  | (50,000) | 0 |  |
Notional Amount of Derivative Instruments Added |  |  |  |  |  |  |  |  |  |  | 470,000 |  |  |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off |  |  |  |  |  |  |  |  |  |  | (420,000) |  |  |
Not Designated as Hedging Instrument, Economic Hedge [Member] | Long [Member] | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  | 0 |  |  |  | (100,000) |  |  |  |  | 0 | (100,000) |  |
Notional Amount of Derivative Instruments Added |  |  |  |  |  |  |  |  |  |  | 0 |  |  |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off |  |  |  |  |  |  |  |  |  |  | (100,000) |  |  |
Not Designated as Hedging Instrument, Economic Hedge [Member] | Short [Member] | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  | (4,515,000) |  |  |  | (5,775,000) |  |  |  |  | (4,515,000) | (5,775,000) |  |
Notional Amount of Derivative Instruments Added |  |  |  |  |  |  |  |  |  |  | 1,770,000 |  |  |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off |  |  |  |  |  |  |  |  |  |  | (3,030,000) |  |  |
Not Designated as Hedging Instrument, Economic Hedge [Member] | Short [Member] | TBA securities |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities, fair value |  | 0 |  |  |  | (269) |  |  |  |  | 0 | (269) |  |
Derivative, Notional Amount |  | 0 |  |  |  | (150,000) | [5] |  |  |  | 0 | (150,000) | [5] |
Notional Amount of Derivative Instruments Added |  |  |  |  |  |  |  |  |  |  | 0 |  |  |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off |  |  |  |  |  |  |  |  |  |  | (150,000) |  |  |
Not Designated as Hedging Instrument, Trading | Options on U.S. Treasury futures |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount |  | 0 |  |  |  | 0 |  |  |  |  | 0 | 0 |  |
Notional Amount of Derivative Instruments Added |  |  |  |  |  |  |  |  |  |  | 800,000 |  |  |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off |  |  |  |  |  |  |  |  |  |  | (800,000) |  |  |
Not Designated as Hedging Instrument, Trading | Long [Member] | TBA securities |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative assets, fair value |  | 6,239 |  |  |  | 1,483 |  |  |  |  | 6,239 | 1,483 |  |
Derivative, Notional Amount | [5] | (860,000) |  |  |  | (795,000) |  |  |  |  | (860,000) | (795,000) |  |
Notional Amount of Derivative Instruments Added |  |  |  |  |  |  |  |  |  |  | 9,689,000 |  |  |
Notional Amount of Derivative Instruments Maturing, Settled, Terminated, or Paired-Off |  |  |  |  |  |  |  |  |  |  | (9,624,000) |  |  |
Not Designated as Hedging Instrument, Trading | Short [Member] | U.S. Treasury futures |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative liabilities, fair value |  | (1,218) |  |  |  | 0 |  |  |  |  | (1,218) | 0 |  |
Maturity in three years or less | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount | [1] | $ (1,560,000) |  |  |  | $ (3,320,000) |  |  |  |  | $ (1,560,000) | $ (3,320,000) |  |
Derivative, Average Fixed Interest Rate |  | 1.96% |  |  |  | 1.35% |  |  |  |  | 1.96% | 1.35% |  |
Derivative, Average Remaining Maturity |  |  |  |  |  |  |  |  |  |  | 1 year 4 months 24 days | 21 days |  |
Derivative, Fair Value, Net | [3] | $ 324 |  |  |  | $ 791 |  |  |  |  | $ 324 | $ 791 |  |
Maturity between 3 and 6 years | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount | [1] | $ (1,230,000) |  |  |  | $ (1,210,000) |  |  |  |  | $ (1,230,000) | $ (1,210,000) |  |
Derivative, Average Fixed Interest Rate |  | 2.23% |  |  |  | 2.00% |  |  |  |  | 2.23% | 2.00% |  |
Derivative, Average Remaining Maturity |  |  |  |  |  |  |  |  |  |  | 4 years 4 months 24 days | 4 years 7 months 6 days |  |
Derivative, Fair Value, Net | [3] | $ 0 |  |  |  | $ 0 |  |  |  |  | $ 0 | $ 0 |  |
Maturity between 6 and 10 years | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount | [1] | $ (1,505,000) |  |  |  | $ (1,025,000) |  |  |  |  | $ (1,505,000) | $ (1,025,000) |  |
Derivative, Average Fixed Interest Rate |  | 2.80% |  |  |  | 2.49% |  |  |  |  | 2.80% | 2.49% |  |
Derivative, Average Remaining Maturity |  |  |  |  |  |  |  |  |  |  | 8 years 3 months 18 days | 8 years |  |
Derivative, Fair Value, Net | [3] | $ 0 |  |  |  | $ 0 |  |  |  |  | $ 0 | $ 0 |  |
Maturity greater than 10 years [Member] | Not Designated as Hedging Instrument, Economic Hedge [Member] | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative, Notional Amount | [1] | $ (220,000) |  |  |  | $ (120,000) |  |  |  |  | $ (220,000) | $ (120,000) |  |
Derivative, Average Fixed Interest Rate |  | 2.81% |  |  |  | 2.75% |  |  |  |  | 2.81% | 2.75% |  |
Derivative, Average Remaining Maturity |  |  |  |  |  |  |  |  |  |  | 21 years 10 months 24 days | 17 years 3 months 18 days |  |
Derivative, Fair Value, Net | [3] | $ 0 |  |  |  | $ 0 |  |  |  |  | $ 0 | $ 0 |  |
[1] | 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 $775,000 and $2,655,000 of pay-fixed forward starting interest rate swaps as of December 31, 2018 and December 31, 2017, respectively.
[2] | Excluding forward starting pay-fixed interest rate swaps, the weighted average pay rate was 2.29% and 1.36% as of December 31, 2018 and December 31, 2017, respectively.
[3] | The majority of the Company’s interest rate swap agreements are centrally cleared through the CME. Please refer to Note 1 for information regarding the exchange of variation margin being legally considered as settlement of the derivative as opposed to a pledge of collateral.
[4] | The Eurodollar futures notional amounts represent the total notional of the 3-month contracts all of which expired in 2018. The maximum notional outstanding for any future 3-month period did not exceed $650,000 during the period indicated.
[5] | Notional amount represents the par value (or principal balance) of the underlying Agency MBS as if settled as of the date indicated.

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