Securities, at Fair Value (Narrative) (Details) - USD ($)		3 Months Ended			6 Months Ended
	Dec. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	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	$ 54,261,000	$ 136,589,000	$ 218,011,000	$ 54,261,000	$ 136,589,000	$ 63,244,000	$ 3,025,000
Impairment and other losses on securities available-for-sale and other assets		0	5,094,000		0	424,745,000
Securities, at fair value
Debt Securities, Available-for-sale [Line Items]
Securities, at fair value	239,000,000.0	196,500,000			196,500,000
Amortized costs	184,900,000	147,200,000			147,200,000
Gross unrealized gains	$ 54,000,000.0	49,300,000			$ 49,300,000
Weighted average yield	12.30%				12.70%
Weighted average to maturity	8 years 8 months 12 days				6 years 3 months 18 days
Proceeds from sale of MSR			574,900,000			711,700,000
Realized losses			$ 53,300,000			28,700,000
Impairment				$ 280,800,000
Non-Agency MBS
Debt Securities, Available-for-sale [Line Items]
Securities, at fair value		0			$ 0
Allowance for loan loss		$ 0			$ 0
MBS and MSR-Related Assets
Debt Securities, Available-for-sale [Line Items]
Impairment and other 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