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Investments in Debt Securities Investments in Debt Securities (Details) - USD ($)
$ in Thousands
12 Months Ended
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).