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Securities, at Fair Value (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 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	$ 63,244,000	$ 218,011,000	$ 86,833,000	$ 3,025,000
Impairment and other losses on securities available-for-sale and other assets	0	419,651,000
Securities, at fair value
Debt Securities, Available-for-sale [Line Items]
Securities, at fair value	244,700,000		239,000,000.0
Amortized costs	185,600,000		184,900,000
Gross unrealized gains	$ 59,100,000		$ 54,000,000.0
Weighted average yield	12.10%		12.30%
Weighted average to maturity	8 years 2 months 12 days		8 years 8 months 12 days
Proceeds from sale of MSR	$ 136,800,000
Realized losses	24,600,000
Impairment	280,800,000
Non-Agency MBS
Debt Securities, Available-for-sale [Line Items]
Allowance for loan loss	$ 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

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Financial Instruments, Owned, Mortgages, Mortgage-backed and Asset-backed Securities, Amortized Costs

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Represents the minimum period of the fixed rate mortgages which act as security for mortgage backed securities in the entity's portfolio.

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The fair value as of the balance sheet date of firm holdings in mortgages, mortgage backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the mortgage backed security) and asset-backed securities that are primarily paid from the cash flows of a discrete pool of receivables or other financial assets (such as credit-card receivables), either fixed or revolving, that by their terms convert into cash within a finite time period. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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The cash inflow from the sale of servicing rights, which contractually entitle the servicer to receive fees and ancillary revenues for performing billing, collection, disbursement and recordkeeping services in connection with a mortgage portfolio. Rights may be obtained via (1) acquisition or assumption of a servicing obligation that does not relate to financial assets of the servicer or its consolidated affiliates; or (2) by originating mortgage loans and then (a) transferring the loans to a Variable Interest Entity (VIE) in a transaction that meets the necessary transfer and classification requirements, or (b) transferring the loans in a transaction that meets the requirements for sale accounting.

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