Mortgage-Backed Securities - Summary of Present Amortized Cost, Fair Value and Unrealized Gain/Losses of Company's MBS Investments (Details) - USD ($)	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Principal or Notional Value	$ 11,002,568,000		$ 12,211,298,000
Total Premium	489,820,000		431,601,000
Total Discount	(988,628,000)		(1,121,721,000)
Amortized Cost	2,708,929,000		3,303,310,000
Allowance for credit losses	(508,000)	$ (54,000)	(180,000)	$ (1,817,000)	$ (6,314,000)	$ 0
Fair Value	3,232,995,000		3,981,820,000
Gross Unrealized Gains	639,546,000		801,000,000
Total Gross Unrealized Loss	(114,972,000)		(122,310,000)
Net Unrealized Gain/(Loss)	524,574,000		678,690,000
Non-Agency RMBS - Senior
Investment Holdings [Line Items]
Amortized Cost	37,989,049		46,500,000
Residential Mortgage-Backed Securities | Non-Agency RMBS - Senior
Investment Holdings [Line Items]
Principal or Notional Value	1,405,783,000		1,560,135,000
Total Premium	3,939,000		3,934,000
Total Discount	(711,225,000)		(773,804,000)
Amortized Cost	698,497,000		790,265,000
Allowance for credit losses	(508,000)		(180,000)
Fair Value	1,116,871,000		1,277,800,000
Gross Unrealized Gains	418,984,000		487,963,000
Total Gross Unrealized Loss	(102,000)		(248,000)
Net Unrealized Gain/(Loss)	418,882,000		487,715,000
Residential Mortgage-Backed Securities | Non-Agency RMBS - Subordinated
Investment Holdings [Line Items]
Principal or Notional Value	827,198,000		905,674,000
Total Premium	6,093,000		7,059,000
Total Discount	(276,574,000)		(347,056,000)
Amortized Cost	556,717,000		565,677,000
Allowance for credit losses	0		0
Fair Value	604,208,000		610,655,000
Gross Unrealized Gains	71,724,000		83,007,000
Total Gross Unrealized Loss	(24,233,000)		(38,029,000)
Net Unrealized Gain/(Loss)	47,491,000		44,978,000
Residential Mortgage-Backed Securities | Non-Agency RMBS - Senior interest-only
Investment Holdings [Line Items]
Principal or Notional Value	4,217,507,000		5,628,240,000
Total Premium	213,510,000		249,610,000
Total Discount	0		0
Amortized Cost	213,510,000		249,610,000
Allowance for credit losses	0		0
Fair Value	198,589,000		262,259,000
Gross Unrealized Gains	41,498,000		67,868,000
Total Gross Unrealized Loss	(56,419,000)		(55,219,000)
Net Unrealized Gain/(Loss)	(14,921,000)		12,649,000
Residential Mortgage-Backed Securities | Agency RMBS - Interest-only
Investment Holdings [Line Items]
Principal or Notional Value	1,164,844,000		1,262,963,000
Total Premium	110,275,000		118,867,000
Total Discount	0		0
Amortized Cost	110,275,000		118,867,000
Allowance for credit losses	0		0
Fair Value	76,820,000		90,738,000
Gross Unrealized Gains	219,000		270,000
Total Gross Unrealized Loss	(33,674,000)		(28,399,000)
Net Unrealized Gain/(Loss)	(33,455,000)		(28,129,000)
Commercial Mortgage Backed Securities | Agency MBS - Commercial
Investment Holdings [Line Items]
Principal or Notional Value	974,756,000		1,527,621,000
Total Premium	18,194,000		28,559,000
Total Discount	(829,000)		(861,000)
Amortized Cost	992,121,000		1,555,319,000
Allowance for credit losses	0		0
Fair Value	1,094,145,000		1,714,483,000
Gross Unrealized Gains	102,031,000		159,233,000
Total Gross Unrealized Loss	(7,000)		(69,000)
Net Unrealized Gain/(Loss)	102,024,000		159,164,000
Commercial Mortgage Backed Securities | Agency CMBS - Interest-only
Investment Holdings [Line Items]
Principal or Notional Value	2,412,480,000		1,326,665,000
Total Premium	137,809,000		23,572,000
Total Discount	0		0
Amortized Cost	137,809,000		23,572,000
Allowance for credit losses	0		0
Fair Value	142,362,000		25,885,000
Gross Unrealized Gains	5,090,000		2,659,000
Total Gross Unrealized Loss	(537,000)		(346,000)
Net Unrealized Gain/(Loss)	$ 4,553,000		$ 2,313,000