Securities, at Fair Value (Narrative) (Details) - USD ($)		3 Months Ended			9 Months Ended
	Dec. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Minimum term of fixed rate mortgages underlying MBS					15 years
Maximum term of fixed rate mortgages underlying MBS					30 years
Allowance for loan loss	$ 86,833,000	$ 44,102,000	$ 106,246,000	$ 218,011,000	$ 44,102,000	$ 106,246,000	$ 54,261,000	$ 63,244,000	$ 136,589,000	$ 3,025,000
Impairment and other gains and losses on securities available-for-sale and other assets		(10,000,000)	$ 221,000		(10,000,000)	424,966,000
Securities, at fair value
Debt Securities, Available-for-sale [Line Items]
Securities, at fair value	239,000,000	177,500,000			177,500,000
Amortized costs	184,900,000	135,100,000			135,100,000
Gross unrealized gains	$ 54,000,000	42,500,000			$ 42,500,000
Weighted average yield	12.30%				12.60%
Weighted average to maturity	8 years 8 months 12 days				6 years 1 month 6 days
Proceeds from sale of MSR						711,700,000
Realized losses						28,700,000
Impairment				$ 280,800,000
Non-Agency MBS
Debt Securities, Available-for-sale [Line Items]
Securities, at fair value							$ 0
Allowance for loan loss		$ 0			$ 0
MBS and MSR-Related Assets
Debt Securities, Available-for-sale [Line Items]
Impairment and other gains and losses on securities available-for-sale and other assets						$ 63,500,000
Minimum | RPL/NPL MBS
Debt Securities, Available-for-sale [Line Items]
Debt instrument, basis spread on variable rate					3.00%
Debt instrument, coupon step-up period					36 months
Maximum | RPL/NPL MBS
Debt Securities, Available-for-sale [Line Items]
Debt instrument, basis spread on variable rate					4.00%
Debt instrument, coupon step-up period					48 months