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Investments in Debt Securities Investments in Debt Securities (Details)
12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Investments [Line Items]      
Par value of available for sale debt security $ 2,076,346,000 $ 4,455,808,000  
Investments, Unamortized Premium (Discount) 439,649,000 558,549,000  
Debt Securities, Available-for-sale, Amortized Cost 2,515,995,000 5,014,357,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 81,293,000 175,157,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax (1,033,000) (1,351,000)  
Debt Securities, Available-for-sale 2,596,255,000 5,188,163,000  
Proceeds from Sale of Available-for-sale Securities 4,642,305,000 1,033,066,000 $ 733,064,000
Debt Securities, Available-for-sale, Realized Gain (Loss) 308,084,000 (5,755,000) (23,373,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss 0    
Agency MBS      
Investments [Line Items]      
Notional balance for interest only securities 11,277,908,000 13,404,824,000  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 19,266,000 215,792,000  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss $ 399,000 $ 1,139,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year 19 27  
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value $ 749,000 $ 75,745,000  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 133,000 $ 35,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year 2 2  
Interest-Only-Strip      
Investments [Line Items]      
Par value of available for sale debt security $ 0 [1] $ 0 [2]  
Investments, Unamortized Premium (Discount) 378,940,000 488,145,000  
Debt Securities, Available-for-sale, Amortized Cost 378,940,000 488,145,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 12,081,000 11,760,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax (982,000) (863,000)  
Debt Securities, Available-for-sale 390,039,000 499,042,000  
Non-Agency MBS      
Investments [Line Items]      
Par value of available for sale debt security 1,499,000 1,938,000  
Investments, Unamortized Premium (Discount) (440,000) (780,000)  
Notional balance for interest only securities 9,319,520,000 9,799,629,000  
Debt Securities, Available-for-sale, Amortized Cost 1,059,000 1,158,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 267,000 552,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax (51,000) (20,000)  
Debt Securities, Available-for-sale 1,275,000 1,690,000  
Proceeds from Sale of Available-for-sale Securities     8,695,000
Debt Securities, Available-for-sale, Realized Loss     51,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value 33,417,000 13,607,000  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss $ 408,000 $ 146,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year 23 7  
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value $ 2,156,000 $ 1,099,000  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 93,000 $ 31,000  
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year 5 5  
U.S. Treasuries      
Investments [Line Items]      
Proceeds from Sale of Available-for-sale Securities     248,803,000
Debt Securities, Available-for-sale, Realized Loss     (6,567,000)
Agency | Commercial Mortgage Backed Securities      
Investments [Line Items]      
Par value of available for sale debt security $ 235,801,000 $ 1,890,186,000  
Investments, Unamortized Premium (Discount) 3,152,000 15,414,000  
Debt Securities, Available-for-sale, Amortized Cost 238,953,000 1,905,600,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 19,597,000 93,763,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 0 (6,000)  
Debt Securities, Available-for-sale 258,550,000 1,999,357,000  
Proceeds from Sale of Available-for-sale Securities 2,247,273,000 213,199,000 242,029,000
Debt Securities, Available-for-sale, Realized Loss 225,395,000 (6,493,000) (9,218,000)
Agency | Residential Mortgage Backed Securities      
Investments [Line Items]      
Par value of available for sale debt security 1,839,046,000 2,563,684,000  
Investments, Unamortized Premium (Discount) 57,997,000 55,770,000  
Debt Securities, Available-for-sale, Amortized Cost 1,897,043,000 2,619,454,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax 49,348,000 69,082,000  
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 0 (462,000)  
Debt Securities, Available-for-sale 1,946,391,000 2,688,074,000  
Proceeds from Sale of Available-for-sale Securities 2,395,032,000 796,699,000 217,837,000
Debt Securities, Available-for-sale, Realized Gain $ 82,689,000    
Debt Securities, Available-for-sale, Realized Loss   506,000 (7,785,000)
Agency | Interest-Only-Strip      
Investments [Line Items]      
Proceeds from Sale of Available-for-sale Securities   23,168,000 15,700,000
Debt Securities, Available-for-sale, Realized Loss   $ 232,000 $ 146,000
[1] The notional balance for Agency CMBS IO and non-Agency CMBS IO was $11,277,908 and $9,319,520 respectively, as of December 31, 2020.
[2] The notional balance for the Agency CMBS IO and non-Agency CMBS IO was $13,404,824 and $9,799,629, respectively, as of December 31, 2019.