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Investments in Debt Securities Investments in Debt Securities (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | $ 4,455,808 |  | $ 3,194,441 |  |
Investments, Unamortized Premium (Discount) |  | 558,549 |  | 590,968 |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 5,014,357 |  | 3,785,409 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 175,157 |  | 21,436 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (1,351) |  | (57,381) |  |
Debt Securities, Available-for-sale |  | 5,188,163 |  | 3,749,464 |  |
Proceeds from Sale of Available-for-sale Securities |  | 1,033,066 |  | 733,064 |  | $ 1,073,093
Debt Securities, Available-for-sale, Realized Gain (Loss) |  | (5,755) |  | (23,373) |  | (11,530)
Agency MBS |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Notional balance for interest only securities |  | 13,404,824 |  | 13,048,666 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 215,792 |  | 581,440 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | $ 1,139 |  | $ 1,793 |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year |  | 27 |  | 28 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | $ 75,745 |  | $ 1,543,892 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | $ 35 |  | $ 54,260 |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year |  | 2 |  | 88 |  |
Non-Agency MBS |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Notional balance for interest only securities |  | $ 9,799,629 |  | $ 10,275,494 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 13,607 |  | 70,876 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | $ 146 |  | $ 581 |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year |  | 7 |  | 22 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | $ 1,099 |  | $ 46,154 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | $ 31 |  | $ 747 |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year |  | 5 |  | 19 |  |
Residential Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | $ 2,564,339 |  | $ 2,119,495 |  |
Investments, Unamortized Premium (Discount) |  | 55,770 |  | 56,744 |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 2,620,109 |  | 2,176,239 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 69,111 |  | 8,926 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (482) |  | (26,286) |  |
Debt Securities, Available-for-sale |  | 2,688,738 |  | 2,158,879 |  |
Commercial Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 1,891,469 |  | 1,074,946 |  |
Investments, Unamortized Premium (Discount) |  | 14,634 |  | 6,481 |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 1,906,103 |  | 1,081,427 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 94,286 |  | 6,554 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (6) |  | (29,550) |  |
Debt Securities, Available-for-sale |  | 2,000,383 |  | 1,058,431 |  |
Interest-Only-Strip |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 0 | [1] | 0 | [2] |
Investments, Unamortized Premium (Discount) |  | 488,145 |  | 527,743 |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 488,145 |  | 527,743 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 11,760 |  | 5,956 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (863) |  | (1,545) |  |
Debt Securities, Available-for-sale |  | 499,042 |  | 532,154 |  |
U.S. Treasuries |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Proceeds from Sale of Available-for-sale Securities |  | 0 |  | 248,803 |  | 51,797
Debt Securities, Available-for-sale, Realized Gain |  | 0 |  |  |  |
Debt Securities, Available-for-sale, Realized Loss |  |  |  | (6,567) |  | (244)
Agency | Residential Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 2,563,684 |  | 2,118,639 |  |
Investments, Unamortized Premium (Discount) |  | 55,770 |  | 56,744 |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 2,619,454 |  | 2,175,383 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 69,082 |  | 8,902 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (462) |  | (26,264) |  |
Debt Securities, Available-for-sale |  | $ 2,688,074 |  | $ 2,158,021 |  |
Investments, Weighted Average Coupon Rate | [3] | 3.76% |  | 3.95% |  |
Proceeds from Sale of Available-for-sale Securities |  | $ 796,699 |  | $ 217,837 |  | 716,560
Debt Securities, Available-for-sale, Realized Gain |  | 506 |  |  |  |
Debt Securities, Available-for-sale, Realized Loss |  |  |  | (7,785) |  | (12,392)
Agency | Commercial Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 1,890,186 |  | 1,071,906 |  |
Investments, Unamortized Premium (Discount) |  | 15,414 |  | 8,518 |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 1,905,600 |  | 1,080,424 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 93,763 |  | 6,141 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (6) |  | (29,550) |  |
Debt Securities, Available-for-sale |  | $ 1,999,357 |  | $ 1,057,015 |  |
Investments, Weighted Average Coupon Rate | [3] | 3.30% |  | 3.22% |  |
Proceeds from Sale of Available-for-sale Securities |  | $ 213,199 |  | $ 242,029 |  | 252,624
Debt Securities, Available-for-sale, Realized Loss |  | (6,493) |  | (9,218) |  | (135)
Agency | Interest-Only-Strip |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 0 | [1] | 0 | [2] |
Investments, Unamortized Premium (Discount) |  | 298,900 |  | 287,062 |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 298,900 |  | 287,062 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 7,431 |  | 4,281 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (706) |  | (239) |  |
Debt Securities, Available-for-sale |  | $ 305,625 |  | $ 291,104 |  |
Investments, Weighted Average Coupon Rate | [3] | 0.65% |  | 0.55% |  |
Proceeds from Sale of Available-for-sale Securities |  | $ 23,168 |  | $ 15,700 |  |
Debt Securities, Available-for-sale, Realized Gain |  | 232 |  | 146 |  |
Non-Agency | Residential Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | 655 |  | 856 |  |
Investments, Unamortized Premium (Discount) |  | 0 |  | 0 |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 655 |  | 856 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 29 |  | 24 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (20) |  | (22) |  |
Debt Securities, Available-for-sale |  | $ 664 |  | $ 858 |  |
Investments, Weighted Average Coupon Rate | [3] | 6.75% |  | 6.75% |  |
Proceeds from Sale of Available-for-sale Securities |  |  |  |  |  | 16,407
Debt Securities, Available-for-sale, Realized Gain |  |  |  |  |  | 42
Non-Agency | Commercial Mortgage Backed Securities |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | $ 1,283 |  | $ 3,040 |  |
Investments, Unamortized Premium (Discount) |  | (780) |  | (2,037) |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 503 |  | 1,003 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 523 |  | 413 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 0 |  | 0 |  |
Debt Securities, Available-for-sale |  | $ 1,026 |  | $ 1,416 |  |
Investments, Weighted Average Coupon Rate | [3] | 7.06% |  | 6.47% |  |
Proceeds from Sale of Available-for-sale Securities |  |  |  |  |  | 35,705
Debt Securities, Available-for-sale, Realized Gain |  |  |  |  |  | $ 1,199
Non-Agency | Interest-Only-Strip |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Par value of available for sale debt security |  | $ 0 | [1] | $ 0 | [2] |
Investments, Unamortized Premium (Discount) |  | 189,245 |  | 240,681 |  |
Debt Securities, Available-for-sale, Amortized Cost |  | 189,245 |  | 240,681 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 4,329 |  | 1,675 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | (157) |  | (1,306) |  |
Debt Securities, Available-for-sale |  | $ 193,417 |  | $ 241,050 |  |
Investments, Weighted Average Coupon Rate | [3] | 0.62% |  | 0.57% |  |
Proceeds from Sale of Available-for-sale Securities |  | $ 0 |  | $ 8,695 |  |
Debt Securities, Available-for-sale, Realized Gain |  | $ 0 |  | $ 51 |  |
[1] | The notional balance for Agency CMBS IO and non-Agency CMBS IO was $13,404,824 and $9,799,629 respectively, as of December 31, 2019.
[2] | The notional balance for the Agency CMBS IO and non-Agency CMBS IO was $13,048,666 and $10,275,494, respectively, as of December 31, 2018.
[3] | The weighted average coupon (“WAC”) is the gross interest rate of the security weighted by the outstanding principal balance (or by notional balance in the case of an IO security).

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- Definition This item represents the current weighted-average gross interest rate earned on this type of MBS in the investment portfolio as of the period indicated. The gross interest rate of each MBS within the portfolio is weighted based on its current outstanding notional or par balance. The current outstanding notional or par balance for each MBS consists of the total of all principal balances remaining on the mortgage loans underlying each MBS. + References No definition available. + Details
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- Definition Notional balance for interest only securities owned by the Company. Interest only securities represent the right to receive excess interest payments, and not principal cash flows, based on the underlying unpaid principal balance (ie, notional) of the underlying pool of mortgage loans. + References No definition available. + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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